From 36d22d82aa202bb199967e9512281e9a53db42c9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sun, 7 Apr 2024 21:33:14 +0200
Subject: Adding upstream version 115.7.0esr.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../cssom/CSS-namespace-object-class-string.html   |  47 ++
 .../css/cssom/CSSConditionRule-conditionText.html  |  23 +
 .../css/cssom/CSSContainerRule.tentative.html      |  35 +
 .../tests/css/cssom/CSSCounterStyleRule.html       |  21 +
 .../tests/css/cssom/CSSFontFaceRule.html           |  21 +
 .../tests/css/cssom/CSSFontFeatureValuesRule.html  | 192 +++++
 .../tests/css/cssom/CSSGroupingRule-cssRules.html  |  54 ++
 .../css/cssom/CSSGroupingRule-insertRule.html      | 118 +++
 .../tests/css/cssom/CSSKeyframeRule.html           |  52 ++
 .../tests/css/cssom/CSSKeyframesRule.html          |  85 +++
 .../tests/css/cssom/CSSNamespaceRule.html          |  32 +
 .../web-platform/tests/css/cssom/CSSRuleList.html  |  27 +
 .../CSSStyleRule-set-selectorText-namespace.html   |  60 ++
 .../css/cssom/CSSStyleRule-set-selectorText.html   | 158 ++++
 .../web-platform/tests/css/cssom/CSSStyleRule.html | 113 +++
 ...StyleSheet-constructable-baseURL.tentative.html |  66 ++
 .../CSSStyleSheet-constructable-concat-ref.html    |  12 +
 .../cssom/CSSStyleSheet-constructable-concat.html  |  28 +
 .../CSSStyleSheet-constructable-cssRules.html      |  22 +
 ...tructable-disabled-regular-sheet-insertion.html |  22 +
 ...et-constructable-disallow-import.tentative.html |  83 +++
 .../CSSStyleSheet-constructable-duplicate.html     |  84 +++
 ...eet-constructable-replace-on-regular-sheet.html |  42 ++
 .../css/cssom/CSSStyleSheet-constructable.html     | 801 +++++++++++++++++++++
 .../cssom/CSSStyleSheet-modify-after-removal.html  |  29 +
 .../css/cssom/CSSStyleSheet-template-adoption.html |  58 ++
 .../tests/css/cssom/CSSStyleSheet.html             | 132 ++++
 .../css/cssom/HTMLLinkElement-disabled-001.html    |  44 ++
 .../css/cssom/HTMLLinkElement-disabled-002.html    |  37 +
 .../css/cssom/HTMLLinkElement-disabled-003.html    |  31 +
 .../css/cssom/HTMLLinkElement-disabled-004.html    |  39 +
 .../css/cssom/HTMLLinkElement-disabled-005.html    |  26 +
 .../css/cssom/HTMLLinkElement-disabled-006.html    |  25 +
 .../css/cssom/HTMLLinkElement-disabled-007.html    |  26 +
 .../HTMLLinkElement-disabled-alternate-ref.html    |   7 +
 .../cssom/HTMLLinkElement-disabled-alternate.html  |  14 +
 testing/web-platform/tests/css/cssom/META.yml      |   5 +
 .../web-platform/tests/css/cssom/MediaList.html    |  55 ++
 .../web-platform/tests/css/cssom/MediaList2.xhtml  |  45 ++
 .../tests/css/cssom/MutationObserver-style.html    |  43 ++
 .../tests/css/cssom/StyleSheetList.html            |  34 +
 .../cssom/adoptedstylesheets-observablearray.html  |  92 +++
 .../web-platform/tests/css/cssom/at-namespace.html |  29 +
 testing/web-platform/tests/css/cssom/base-uri.html |  55 ++
 .../css/cssom/border-shorthand-serialization.html  |  42 ++
 ...caretPositionFromPoint-with-transformation.html |  46 ++
 .../tests/css/cssom/computed-style-001.html        |  66 ++
 .../tests/css/cssom/computed-style-002.html        |  17 +
 .../tests/css/cssom/computed-style-003.html        |  20 +
 .../tests/css/cssom/computed-style-004.html        |  29 +
 .../tests/css/cssom/computed-style-005.html        |  64 ++
 .../css/cssom/computed-style-set-property.html     |  36 +
 .../tests/css/cssom/css-style-attr-decl-block.html | 148 ++++
 .../cssom/css-style-attribute-modifications.html   |  16 +
 .../cssom/css-style-declaration-modifications.html |  71 ++
 .../tests/css/cssom/css-style-reparse.html         |  59 ++
 .../tests/css/cssom/cssimportrule-parent.html      |  20 +
 .../css/cssom/cssimportrule-sheet-identity.html    |  24 +
 .../tests/css/cssom/cssimportrule.html             | 122 ++++
 .../tests/css/cssom/cssom-cssText-serialize.html   |  28 +
 .../css/cssom/cssom-cssstyledeclaration-set.html   |  37 +
 .../css/cssom/cssom-fontfacerule-constructors.html |  63 ++
 .../tests/css/cssom/cssom-fontfacerule.html        |  58 ++
 .../cssom-getPropertyValue-common-checks.html      | 223 ++++++
 .../tests/css/cssom/cssom-pagerule.html            |  43 ++
 .../tests/css/cssom/cssom-ruleTypeAndOrder.html    |  75 ++
 .../css/cssom/cssom-setProperty-shorthand.html     |  77 ++
 .../cssstyledeclaration-cssfontrule.tentative.html |  27 +
 .../cssstyledeclaration-csstext-all-shorthand.html |  38 +
 ...ssstyledeclaration-csstext-final-delimiter.html |  32 +
 .../cssstyledeclaration-csstext-important.html     |  12 +
 .../css/cssom/cssstyledeclaration-csstext.html     | 126 ++++
 .../cssstyledeclaration-custom-properties.html     |  19 +
 .../css/cssom/cssstyledeclaration-mutability.html  |  69 ++
 .../cssstyledeclaration-mutationrecord-001.html    |  20 +
 .../cssstyledeclaration-mutationrecord-002.html    |  15 +
 .../cssstyledeclaration-mutationrecord-003.html    |  16 +
 .../cssstyledeclaration-mutationrecord-004.html    |  16 +
 .../cssstyledeclaration-mutationrecord-005.html    |  16 +
 .../css/cssom/cssstyledeclaration-properties.html  |  15 +
 ...ledeclaration-registered-custom-properties.html |  58 ++
 .../css/cssom/cssstyledeclaration-setter-attr.html |  18 +
 .../cssstyledeclaration-setter-declarations.html   | 160 ++++
 .../cssstyledeclaration-setter-form-controls.html  | 103 +++
 .../cssom/cssstyledeclaration-setter-logical.html  |  73 ++
 .../css/cssom/declaration-block-all-crash.html     |  10 +
 testing/web-platform/tests/css/cssom/escape.html   | 102 +++
 .../tests/css/cssom/flex-serialization.html        |  36 +
 .../css/cssom/font-family-serialization-001.html   |  44 ++
 .../css/cssom/font-shorthand-serialization.html    |  17 +
 .../font-variant-shorthand-serialization.html      | 131 ++++
 ...tedStyle-animations-replaced-into-ib-split.html |  34 +
 .../cssom/getComputedStyle-detached-subtree.html   |  49 ++
 .../cssom/getComputedStyle-display-none-001.html   |  39 +
 .../cssom/getComputedStyle-display-none-002.html   |  34 +
 .../cssom/getComputedStyle-display-none-003.html   |  26 +
 .../css/cssom/getComputedStyle-dynamic-subdoc.html |  33 +
 ...omputedStyle-getter-v-properties.tentative.html |  34 +
 .../cssom/getComputedStyle-insets-absolute.html    |  21 +
 .../css/cssom/getComputedStyle-insets-fixed.html   |  21 +
 .../css/cssom/getComputedStyle-insets-nobox.html   |  18 +
 .../cssom/getComputedStyle-insets-relative.html    |  19 +
 .../css/cssom/getComputedStyle-insets-static.html  |  19 +
 ...edStyle-insets-sticky-container-for-abspos.html |  26 +
 .../css/cssom/getComputedStyle-insets-sticky.html  |  22 +
 ...dStyle-layout-dependent-removed-ib-sibling.html |  30 +
 ...le-layout-dependent-replaced-into-ib-split.html |  30 +
 .../css/cssom/getComputedStyle-line-height.html    |  23 +
 .../getComputedStyle-logical-enumeration.html      |  27 +
 .../css/cssom/getComputedStyle-property-order.html |  26 +
 .../tests/css/cssom/getComputedStyle-pseudo.html   | 153 ++++
 .../cssom/getComputedStyle-resolved-colors.html    |  42 ++
 ...getComputedStyle-resolved-min-max-clamping.html |  38 +
 .../cssom/getComputedStyle-sticky-pos-percent.html |  22 +
 .../getComputedStyle-width-scroll.tentative.html   |  26 +
 .../web-platform/tests/css/cssom/historical.html   |  58 ++
 .../web-platform/tests/css/cssom/idlharness.html   |  77 ++
 .../tests/css/cssom/inline-style-001.html          |  78 ++
 .../tests/css/cssom/insertRule-across-context.html |  32 +
 .../css/cssom/insertRule-charset-no-index.html     |  32 +
 .../css/cssom/insertRule-from-script-ref.html      |   6 +
 .../tests/css/cssom/insertRule-from-script.html    |  12 +
 .../css/cssom/insertRule-import-no-index.html      |  33 +
 .../css/cssom/insertRule-namespace-no-index.html   |  40 +
 .../tests/css/cssom/insertRule-no-index.html       |  35 +
 .../css/cssom/insertRule-syntax-error-01.html      |  14 +
 .../tests/css/cssom/medialist-dynamic-001-ref.html |   5 +
 .../tests/css/cssom/medialist-dynamic-001.html     |  12 +
 .../tests/css/cssom/medialist-interfaces-001.html  |  83 +++
 .../tests/css/cssom/medialist-interfaces-002.html  |  71 ++
 .../tests/css/cssom/medialist-interfaces-004.html  |  64 ++
 .../tests/css/cssom/mediaquery-sort-dedup.html     |  16 +
 .../tests/css/cssom/overflow-serialization.html    |  61 ++
 .../tests/css/cssom/page-descriptors.html          |  40 +
 .../css/cssom/preferred-stylesheet-order.html      |  22 +
 .../cssom/preferred-stylesheet-reversed-order.html |  22 +
 .../tests/css/cssom/property-accessors.html        |  64 ++
 .../css/cssom/removerule-invalidation-crash.html   |  16 +
 .../tests/css/cssom/rule-restrictions.html         |  51 ++
 .../tests/css/cssom/selectorSerialize.html         | 121 ++++
 .../selectorText-modification-restyle-001-ref.html |  13 +
 .../selectorText-modification-restyle-001.html     |  22 +
 .../selectorText-modification-restyle-002.html     |  31 +
 ...erialization-CSSDeclaration-with-important.html |  32 +
 .../tests/css/cssom/serialize-all-longhands.html   |  36 +
 .../tests/css/cssom/serialize-custom-props.html    |  69 ++
 .../tests/css/cssom/serialize-media-rule.html      | 185 +++++
 .../cssom/serialize-namespaced-type-selectors.html | 257 +++++++
 .../tests/css/cssom/serialize-values.html          | 620 ++++++++++++++++
 .../css/cssom/serialize-variable-reference.html    |  36 +
 .../css/cssom/setproperty-null-undefined.html      |  47 ++
 .../tests/css/cssom/shorthand-serialization.html   |  86 +++
 .../tests/css/cssom/shorthand-values.html          |  51 ++
 .../cssom/style-attr-update-across-documents.html  |  48 ++
 .../css/cssom/style-sheet-interfaces-001.html      | 140 ++++
 .../css/cssom/style-sheet-interfaces-002.html      |  40 +
 .../cssom/stylesheet-replacedata-dynamic-ref.html  |   7 +
 .../css/cssom/stylesheet-replacedata-dynamic.html  |  12 +
 .../tests/css/cssom/stylesheet-same-origin.css     |   3 +
 .../css/cssom/stylesheet-same-origin.sub.html      |  83 +++
 .../tests/css/cssom/stylesheet-title.html          |  39 +
 .../tests/css/cssom/support/1x1-green.png          | Bin 0 -> 135 bytes
 .../tests/css/cssom/support/1x1-lime.png           | Bin 0 -> 135 bytes
 .../tests/css/cssom/support/1x1-maroon.png         | Bin 0 -> 109 bytes
 .../tests/css/cssom/support/1x1-navy.png           | Bin 0 -> 109 bytes
 .../tests/css/cssom/support/1x1-red.png            | Bin 0 -> 135 bytes
 .../tests/css/cssom/support/1x1-white.png          | Bin 0 -> 109 bytes
 .../tests/css/cssom/support/60x60-gg-rr.png        | Bin 0 -> 224 bytes
 .../tests/css/cssom/support/60x60-green.png        | Bin 0 -> 218 bytes
 .../web-platform/tests/css/cssom/support/README    |  28 +
 .../tests/css/cssom/support/a-green.css            |   1 +
 .../tests/css/cssom/support/b-green.css            |   1 +
 .../web-platform/tests/css/cssom/support/black.css |   4 +
 .../web-platform/tests/css/cssom/support/c-red.css |   1 +
 .../web-platform/tests/css/cssom/support/cat.png   | Bin 0 -> 1883 bytes
 .../css/cssom/support/constructable-import.css     |   3 +
 .../css/cssom/support/getComputedStyle-insets.js   | 375 ++++++++++
 .../tests/css/cssom/support/import-charset.css     |   1 +
 .../tests/css/cssom/support/import-green.css       |   1 +
 .../tests/css/cssom/support/import-red.css         |   1 +
 .../tests/css/cssom/support/import-rule.css        |   1 +
 .../css/cssom/support/malformed-http-response.asis |   1 +
 .../css/cssom/support/pattern-grg-rgr-grg.png      | Bin 0 -> 222 bytes
 .../css/cssom/support/pattern-grg-rrg-rgg.png      | Bin 0 -> 231 bytes
 .../css/cssom/support/pattern-rgr-grg-rgr.png      | Bin 0 -> 223 bytes
 .../tests/css/cssom/support/pattern-tr.png         | Bin 0 -> 137 bytes
 .../tests/css/cssom/support/ruler-h-50%.png        | Bin 0 -> 691 bytes
 .../tests/css/cssom/support/ruler-h-50px.png       | Bin 0 -> 671 bytes
 .../tests/css/cssom/support/ruler-v-100px.png      | Bin 0 -> 760 bytes
 .../tests/css/cssom/support/ruler-v-50px.png       | Bin 0 -> 757 bytes
 .../tests/css/cssom/support/square-purple.png      | Bin 0 -> 92 bytes
 .../tests/css/cssom/support/square-teal.png        | Bin 0 -> 92 bytes
 .../tests/css/cssom/support/square-white.png       | Bin 0 -> 78 bytes
 .../tests/css/cssom/support/support/README         |   4 +
 .../css/cssom/support/support/swatch-green.png     | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/support/swatch-red.png | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/swatch-blue.png        | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/swatch-green.png       | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/swatch-lime.png        | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/swatch-orange.png      | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/swatch-red.png         | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/swatch-teal.png        | Bin 0 -> 156 bytes
 .../tests/css/cssom/support/swatch-white.png       | Bin 0 -> 85 bytes
 .../tests/css/cssom/support/swatch-yellow.png      | Bin 0 -> 84 bytes
 .../tests/css/cssom/support/test-bl.png            | Bin 0 -> 1368 bytes
 .../tests/css/cssom/support/test-br.png            | Bin 0 -> 1045 bytes
 .../css/cssom/support/test-inner-half-size.png     | Bin 0 -> 180 bytes
 .../tests/css/cssom/support/test-outer.png         | Bin 0 -> 2412 bytes
 .../tests/css/cssom/support/test-tl.png            | Bin 0 -> 1025 bytes
 .../tests/css/cssom/support/test-tr.png            | Bin 0 -> 1235 bytes
 .../tests/css/cssom/support/xmlss-pi.xhtml         |   1 +
 .../css/cssom/ttwf-cssom-doc-ext-load-count.html   |  61 ++
 .../cssom/ttwf-cssom-doc-ext-load-tree-order.html  |  56 ++
 .../css/cssom/ttwf-cssom-document-extension.html   |  25 +
 .../tests/css/cssom/variable-names.html            |  48 ++
 .../css/cssom/xml-stylesheet-pi-in-doctype.xhtml   |  17 +
 216 files changed, 9945 insertions(+)
 create mode 100644 testing/web-platform/tests/css/cssom/CSS-namespace-object-class-string.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSConditionRule-conditionText.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSContainerRule.tentative.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSCounterStyleRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSFontFaceRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSFontFeatureValuesRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSGroupingRule-cssRules.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSGroupingRule-insertRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSKeyframeRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSKeyframesRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSNamespaceRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSRuleList.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText-namespace.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleRule.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-baseURL.tentative.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat-ref.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-cssRules.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disabled-regular-sheet-insertion.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disallow-import.tentative.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-duplicate.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-replace-on-regular-sheet.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-modify-after-removal.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet-template-adoption.html
 create mode 100644 testing/web-platform/tests/css/cssom/CSSStyleSheet.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-001.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-002.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-003.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-004.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-005.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-006.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-007.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate-ref.html
 create mode 100644 testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate.html
 create mode 100644 testing/web-platform/tests/css/cssom/META.yml
 create mode 100644 testing/web-platform/tests/css/cssom/MediaList.html
 create mode 100644 testing/web-platform/tests/css/cssom/MediaList2.xhtml
 create mode 100644 testing/web-platform/tests/css/cssom/MutationObserver-style.html
 create mode 100644 testing/web-platform/tests/css/cssom/StyleSheetList.html
 create mode 100644 testing/web-platform/tests/css/cssom/adoptedstylesheets-observablearray.html
 create mode 100644 testing/web-platform/tests/css/cssom/at-namespace.html
 create mode 100644 testing/web-platform/tests/css/cssom/base-uri.html
 create mode 100644 testing/web-platform/tests/css/cssom/border-shorthand-serialization.html
 create mode 100644 testing/web-platform/tests/css/cssom/caretPositionFromPoint-with-transformation.html
 create mode 100644 testing/web-platform/tests/css/cssom/computed-style-001.html
 create mode 100644 testing/web-platform/tests/css/cssom/computed-style-002.html
 create mode 100644 testing/web-platform/tests/css/cssom/computed-style-003.html
 create mode 100644 testing/web-platform/tests/css/cssom/computed-style-004.html
 create mode 100644 testing/web-platform/tests/css/cssom/computed-style-005.html
 create mode 100644 testing/web-platform/tests/css/cssom/computed-style-set-property.html
 create mode 100644 testing/web-platform/tests/css/cssom/css-style-attr-decl-block.html
 create mode 100644 testing/web-platform/tests/css/cssom/css-style-attribute-modifications.html
 create mode 100644 testing/web-platform/tests/css/cssom/css-style-declaration-modifications.html
 create mode 100644 testing/web-platform/tests/css/cssom/css-style-reparse.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssimportrule-parent.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssimportrule-sheet-identity.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssimportrule.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-cssText-serialize.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-cssstyledeclaration-set.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-fontfacerule-constructors.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-fontfacerule.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-getPropertyValue-common-checks.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-pagerule.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-ruleTypeAndOrder.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssom-setProperty-shorthand.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-cssfontrule.tentative.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-all-shorthand.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-final-delimiter.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-important.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-custom-properties.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-mutability.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-001.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-002.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-003.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-004.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-005.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-properties.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-registered-custom-properties.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-attr.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-declarations.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-form-controls.html
 create mode 100644 testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-logical.html
 create mode 100644 testing/web-platform/tests/css/cssom/declaration-block-all-crash.html
 create mode 100644 testing/web-platform/tests/css/cssom/escape.html
 create mode 100644 testing/web-platform/tests/css/cssom/flex-serialization.html
 create mode 100644 testing/web-platform/tests/css/cssom/font-family-serialization-001.html
 create mode 100644 testing/web-platform/tests/css/cssom/font-shorthand-serialization.html
 create mode 100644 testing/web-platform/tests/css/cssom/font-variant-shorthand-serialization.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-animations-replaced-into-ib-split.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-detached-subtree.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-display-none-001.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-display-none-002.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-display-none-003.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-dynamic-subdoc.html
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 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-insets-static.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-insets-sticky-container-for-abspos.html
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 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-line-height.html
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 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-pseudo.html
 create mode 100644 testing/web-platform/tests/css/cssom/getComputedStyle-resolved-colors.html
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 create mode 100644 testing/web-platform/tests/css/cssom/historical.html
 create mode 100644 testing/web-platform/tests/css/cssom/idlharness.html
 create mode 100644 testing/web-platform/tests/css/cssom/inline-style-001.html
 create mode 100644 testing/web-platform/tests/css/cssom/insertRule-across-context.html
 create mode 100644 testing/web-platform/tests/css/cssom/insertRule-charset-no-index.html
 create mode 100644 testing/web-platform/tests/css/cssom/insertRule-from-script-ref.html
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 create mode 100644 testing/web-platform/tests/css/cssom/insertRule-import-no-index.html
 create mode 100644 testing/web-platform/tests/css/cssom/insertRule-namespace-no-index.html
 create mode 100644 testing/web-platform/tests/css/cssom/insertRule-no-index.html
 create mode 100644 testing/web-platform/tests/css/cssom/insertRule-syntax-error-01.html
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 create mode 100644 testing/web-platform/tests/css/cssom/mediaquery-sort-dedup.html
 create mode 100644 testing/web-platform/tests/css/cssom/overflow-serialization.html
 create mode 100644 testing/web-platform/tests/css/cssom/page-descriptors.html
 create mode 100644 testing/web-platform/tests/css/cssom/preferred-stylesheet-order.html
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 create mode 100644 testing/web-platform/tests/css/cssom/removerule-invalidation-crash.html
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 create mode 100644 testing/web-platform/tests/css/cssom/serialize-custom-props.html
 create mode 100644 testing/web-platform/tests/css/cssom/serialize-media-rule.html
 create mode 100644 testing/web-platform/tests/css/cssom/serialize-namespaced-type-selectors.html
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 create mode 100644 testing/web-platform/tests/css/cssom/style-sheet-interfaces-002.html
 create mode 100644 testing/web-platform/tests/css/cssom/stylesheet-replacedata-dynamic-ref.html
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 create mode 100644 testing/web-platform/tests/css/cssom/stylesheet-same-origin.css
 create mode 100644 testing/web-platform/tests/css/cssom/stylesheet-same-origin.sub.html
 create mode 100644 testing/web-platform/tests/css/cssom/stylesheet-title.html
 create mode 100644 testing/web-platform/tests/css/cssom/support/1x1-green.png
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 create mode 100644 testing/web-platform/tests/css/cssom/support/1x1-navy.png
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 create mode 100644 testing/web-platform/tests/css/cssom/support/60x60-green.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/README
 create mode 100644 testing/web-platform/tests/css/cssom/support/a-green.css
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 create mode 100644 testing/web-platform/tests/css/cssom/support/black.css
 create mode 100644 testing/web-platform/tests/css/cssom/support/c-red.css
 create mode 100644 testing/web-platform/tests/css/cssom/support/cat.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/constructable-import.css
 create mode 100644 testing/web-platform/tests/css/cssom/support/getComputedStyle-insets.js
 create mode 100644 testing/web-platform/tests/css/cssom/support/import-charset.css
 create mode 100644 testing/web-platform/tests/css/cssom/support/import-green.css
 create mode 100644 testing/web-platform/tests/css/cssom/support/import-red.css
 create mode 100644 testing/web-platform/tests/css/cssom/support/import-rule.css
 create mode 100644 testing/web-platform/tests/css/cssom/support/malformed-http-response.asis
 create mode 100644 testing/web-platform/tests/css/cssom/support/pattern-grg-rgr-grg.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/pattern-grg-rrg-rgg.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/pattern-rgr-grg-rgr.png
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 create mode 100644 testing/web-platform/tests/css/cssom/support/ruler-h-50px.png
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 create mode 100644 testing/web-platform/tests/css/cssom/support/square-purple.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/square-teal.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/square-white.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/support/README
 create mode 100644 testing/web-platform/tests/css/cssom/support/support/swatch-green.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/support/swatch-red.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/swatch-blue.png
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 create mode 100644 testing/web-platform/tests/css/cssom/support/test-tl.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/test-tr.png
 create mode 100644 testing/web-platform/tests/css/cssom/support/xmlss-pi.xhtml
 create mode 100644 testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-count.html
 create mode 100644 testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-tree-order.html
 create mode 100644 testing/web-platform/tests/css/cssom/ttwf-cssom-document-extension.html
 create mode 100644 testing/web-platform/tests/css/cssom/variable-names.html
 create mode 100644 testing/web-platform/tests/css/cssom/xml-stylesheet-pi-in-doctype.xhtml

(limited to 'testing/web-platform/tests/css/cssom')

diff --git a/testing/web-platform/tests/css/cssom/CSS-namespace-object-class-string.html b/testing/web-platform/tests/css/cssom/CSS-namespace-object-class-string.html
new file mode 100644
index 0000000000..3145467073
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSS-namespace-object-class-string.html
@@ -0,0 +1,47 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM - Symbol.toStringTag value of CSS namespace object</title>
+<link rel="help" href="https://webidl.spec.whatwg.org/#es-namespaces">
+<link rel="help" href="https://drafts.csswg.org/cssom/#namespacedef-css">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+"use strict";
+
+test(() => {
+  assert_own_property(CSS, Symbol.toStringTag);
+
+  const propDesc = Object.getOwnPropertyDescriptor(CSS, Symbol.toStringTag);
+  assert_equals(propDesc.value, "CSS", "value");
+  assert_equals(propDesc.writable, false, "writable");
+  assert_equals(propDesc.enumerable, false, "enumerable");
+  assert_equals(propDesc.configurable, true, "configurable");
+}, "@@toStringTag exists on the namespace object with the appropriate descriptor");
+
+test(() => {
+  assert_equals(CSS.toString(), "[object CSS]");
+  assert_equals(Object.prototype.toString.call(CSS), "[object CSS]");
+}, "Object.prototype.toString applied to the namespace object");
+
+test(t => {
+  assert_own_property(CSS, Symbol.toStringTag, "Precondition: @@toStringTag on the namespace object");
+  t.add_cleanup(() => {
+    Object.defineProperty(CSS, Symbol.toStringTag, { value: "CSS" });
+  });
+
+  Object.defineProperty(CSS, Symbol.toStringTag, { value: "Test" });
+  assert_equals(CSS.toString(), "[object Test]");
+  assert_equals(Object.prototype.toString.call(CSS), "[object Test]");
+}, "Object.prototype.toString applied after modifying the namespace object's @@toStringTag");
+
+test(t => {
+  assert_own_property(CSS, Symbol.toStringTag, "Precondition: @@toStringTag on the namespace object");
+  t.add_cleanup(() => {
+    Object.defineProperty(CSS, Symbol.toStringTag, { value: "CSS" });
+  });
+
+  assert_true(delete CSS[Symbol.toStringTag]);
+  assert_equals(CSS.toString(), "[object Object]");
+  assert_equals(Object.prototype.toString.call(CSS), "[object Object]");
+}, "Object.prototype.toString applied after deleting @@toStringTag");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSConditionRule-conditionText.html b/testing/web-platform/tests/css/cssom/CSSConditionRule-conditionText.html
new file mode 100644
index 0000000000..bccfc3135a
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSConditionRule-conditionText.html
@@ -0,0 +1,23 @@
+<!doctype html>
+<title>CSSConditionRule.conditionText</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://drafts.csswg.org/css-conditional-3/#cssconditionrule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+@media not all {
+  :root { color: lime }
+}
+</style>
+<script>
+test(function(t) {
+  let rule = document.styleSheets[0].cssRules[0];
+  assert_true(rule instanceof CSSConditionRule);
+  assert_equals(rule.conditionText, "not all");
+  rule.conditionText = 1;
+  assert_equals(rule.conditionText, "not all");
+  rule.conditionText = "all";
+  assert_equals(rule.conditionText, "not all");
+  assert_not_equals(getComputedStyle(document.documentElement).color, "rgb(0, 255, 0)");
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSContainerRule.tentative.html b/testing/web-platform/tests/css/cssom/CSSContainerRule.tentative.html
new file mode 100644
index 0000000000..4e01c0b470
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSContainerRule.tentative.html
@@ -0,0 +1,35 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSContainerRule</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/css-contain-3/#the-csscontainerrule-interface">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style id="sheet"></style>
+<script>
+  const kTests = [
+    {
+      rule: "@container name (min-width: 100px) {}",
+      name: "name",
+      query: "(min-width: 100px)",
+    },
+    {
+      rule: "@container (min-width: 100px) {}",
+      name: "",
+      query: "(min-width: 100px)",
+    },
+  ];
+
+  const sheet = document.getElementById("sheet").sheet;
+  for (let { rule, name, query } of kTests) {
+    test(function() {
+      sheet.insertRule(rule, 0);
+      let r = sheet.cssRules[0];
+      assert_true(r instanceof CSSContainerRule);
+      assert_equals(r.containerName, name, "containerName");
+      assert_equals(r.containerQuery, query, "containerQuery");
+      assert_equals(r.conditionText, name ? `${name} ${query}` : query, "conditionText");
+    }, rule)
+  }
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSCounterStyleRule.html b/testing/web-platform/tests/css/cssom/CSSCounterStyleRule.html
new file mode 100644
index 0000000000..afae84b0b2
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSCounterStyleRule.html
@@ -0,0 +1,21 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM - CSSCounterStyleRule interface</title>
+<link rel="help" href="https://drafts.csswg.org/css-counter-styles-3/#csscounterstylerule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  @counter-style foo {
+    system: cyclic;
+    symbols: ‣;
+    suffix: " ";
+  }
+</style>
+<script>
+test(function() {
+  // TODO: The exact serialization isn't well specified (the ordering of the
+  // descriptors isn't defined).
+  let rule = document.styleSheets[0].cssRules[0];
+  assert_false(rule.cssText.includes('\n'));
+}, "CSSCounterStyleRule.cssText doesn't serialize with newlines");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSFontFaceRule.html b/testing/web-platform/tests/css/cssom/CSSFontFaceRule.html
new file mode 100644
index 0000000000..438c30df9e
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSFontFaceRule.html
@@ -0,0 +1,21 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM -  CSSFontFaceRule interface</title>
+<link rel="help" href="https://drafts.csswg.org/css-fonts-4/#cssfontfacerule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  @font-face {
+    src: local("foo");
+    font-family: foo;
+    font-weight: bold;
+  }
+</style>
+<script>
+test(function() {
+  // TODO: The exact serialization isn't consistent across browsers
+  // (Firefox doesn't preserve ordering of the descriptors, WebKit/Blink does).
+  let rule = document.styleSheets[0].cssRules[0];
+  assert_false(rule.cssText.includes('\n'));
+}, "CSSFontFaceRule.cssText doesn't serialize with newlines");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSFontFeatureValuesRule.html b/testing/web-platform/tests/css/cssom/CSSFontFeatureValuesRule.html
new file mode 100644
index 0000000000..a0ea64a503
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSFontFeatureValuesRule.html
@@ -0,0 +1,192 @@
+<!DOCTYPE html>
+<meta charset="utf-8" />
+<title>CSSOM - CSSFontFeatureValuesRule interface</title>
+<link
+  rel="help"
+  href="https://drafts.csswg.org/css-fonts-4/#om-fontfeaturevalues"
+/>
+<link rel="author" title="Dominik Röttsches" href="drott@chromium.org" />
+<meta
+  name="assert"
+  content="CSSFontFeatureValuesRule interface is accessible and supports read and write access."
+/>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style></style>
+<body></body>
+<script>
+  let rule_initial = `
+  @font-feature-values test_family {
+    @annotation {
+      the_first: 6;
+    }
+    @styleset {
+      yo: 7;
+      del: 4;
+      di: 10 9 4 5;
+    }
+  }
+  `;
+
+  let rule_added = `
+  @font-feature-values second_family {
+    @annotation {
+      yabab: 2;
+      yogog: 4;
+      bidib: 5;
+    }
+  }
+  `;
+
+  let rule_overlap = `
+  @font-feature-values test_family {
+    @annotation {
+      baric: 17;
+    }
+    @ornaments {
+      fooic: 1;
+    }
+  }
+  `;
+
+  function resetStateAndTest(testFunction) {
+    var sheet = document.styleSheets[0];
+    while (sheet.cssRules.length > 0) sheet.deleteRule(0);
+    sheet.insertRule(rule_initial);
+    testFunction();
+  }
+
+  test(function () {
+    resetStateAndTest(() => {
+      let rule = document.styleSheets[0].cssRules[0];
+      assert_equals(rule.type, 14);
+      assert_equals(rule.annotation.size, 1);
+      assert_equals(rule.styleset.size, 3);
+      assert_array_equals(rule.styleset.get("yo"), [7]);
+      assert_array_equals(rule.styleset.get("del"), [4]);
+      assert_array_equals(rule.styleset.get("di"), [10, 9, 4, 5]);
+      assert_array_equals(rule.annotation.get("the_first"), [6]);
+      assert_equals(rule.stylistic.size, 0);
+      assert_equals(rule.swash.size, 0);
+      assert_equals(rule.ornaments.size, 0);
+      assert_equals(rule.characterVariant.size, 0);
+      assert_equals(rule.fontFamily, "test_family");
+    });
+  }, "CSSFontFeatureValuesRule is correctly parsed and accessible via CSSOM.");
+
+  test(function () {
+    resetStateAndTest(() => {
+      let family_override = "test_changed_family";
+      let rule = document.styleSheets[0].cssRules[0];
+      rule.fontFamily = family_override;
+      let read_rule = document.styleSheets[0].cssRules[0];
+      assert_equals(read_rule.fontFamily, family_override);
+    });
+  }, "CSSFontFeatureValuesRule family is settable and readable.");
+
+  test(function () {
+    resetStateAndTest(() => {
+      let rule = document.styleSheets[0].cssRules[0];
+      let override_number = 43;
+      assert_equals(rule.styleset.get("di").length, 4);
+      rule.styleset.set("di", override_number);
+      assert_array_equals(
+        document.styleSheets[0].cssRules[0].styleset.get("di"),
+        [override_number]
+      );
+      assert_equals(rule.styleset.size, 3);
+      assert_equals(
+        document.styleSheets[0].cssRules[0].styleset.get("di").length,
+        1
+      );
+    });
+  }, "CSSFontFeatureValuesMap entries are settable to single values.");
+
+  test(function () {
+    resetStateAndTest(() => {
+      let rule = document.styleSheets[0].cssRules[0];
+      let override_sequence = [43, 22];
+      assert_equals(
+        document.styleSheets[0].cssRules[0].styleset.get("di").length,
+        4
+      );
+      rule.styleset.set("di", override_sequence);
+      assert_array_equals(
+        document.styleSheets[0].cssRules[0].styleset.get("di"),
+        override_sequence
+      );
+      assert_equals(document.styleSheets[0].cssRules[0].styleset.size, 3);
+      assert_equals(
+        document.styleSheets[0].cssRules[0].styleset.get("di").length,
+        2
+      );
+    });
+  }, "CSSFontFeatureValuesMap entries are settable to sequences of numbers.");
+
+  test(function () {
+    resetStateAndTest(() => {
+      document.styleSheets[0].insertRule(rule_added, 1);
+      assert_equals(document.styleSheets[0].cssRules[1].type, 14);
+      assert_equals(document.styleSheets[0].cssRules[1].annotation.size, 3);
+    });
+  }, "New rules can be added.");
+
+  test(function () {
+    resetStateAndTest(() => {
+      document.styleSheets[0].insertRule(rule_added, 1);
+      assert_equals(document.styleSheets[0].cssRules[1].annotation.size, 3);
+      document.styleSheets[0].cssRules[1].annotation.delete("yogog");
+      assert_equals(document.styleSheets[0].cssRules[1].annotation.size, 2);
+    });
+  }, "Deleting single entries is possible.");
+
+  test(function () {
+    resetStateAndTest(() => {
+      document.styleSheets[0].insertRule(rule_added, 1);
+      assert_equals(document.styleSheets[0].cssRules[1].annotation.size, 3);
+      document.styleSheets[0].cssRules[1].annotation.clear();
+      assert_equals(document.styleSheets[0].cssRules[1].annotation.size, 0);
+    });
+  }, "Clearing all entries is possible.");
+
+  test(function () {
+    resetStateAndTest(() => {
+      assert_equals(document.styleSheets[0].cssRules.length, 1);
+      document.styleSheets[0].insertRule(rule_overlap, 1);
+      assert_equals(document.styleSheets[0].cssRules.length, 2);
+
+      // Force updating internal state.
+      getComputedStyle(document.body).borderTop;
+
+      assert_equals(
+        document.styleSheets[0].cssRules[0].annotation.size,
+        1,
+        "1 annotation."
+      );
+      assert_equals(
+        document.styleSheets[0].cssRules[0].styleset.size,
+        3,
+        "3 entries in styleset."
+      );
+      assert_equals(
+        document.styleSheets[0].cssRules[0].ornaments.size,
+        0,
+        "No ornament entries."
+      );
+      assert_false(
+        document.styleSheets[0].cssRules[0].annotation.has("baric"),
+        "Annotation must not contain 'baric'."
+      );
+      assert_false(
+        document.styleSheets[0].cssRules[0].ornaments.has("fooic"),
+        "Ornaments must not contain 'fooic'."
+      );
+
+      assert_equals(document.styleSheets[0].cssRules[1].annotation.size, 1);
+      assert_equals(document.styleSheets[0].cssRules[1].ornaments.size, 1);
+      assert_false(
+        document.styleSheets[0].cssRules[1].annotation.has("the_first")
+      );
+    });
+  }, "Multiple rules for the same family are kept separate in CSSOM.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSGroupingRule-cssRules.html b/testing/web-platform/tests/css/cssom/CSSGroupingRule-cssRules.html
new file mode 100644
index 0000000000..e4afd5e560
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSGroupingRule-cssRules.html
@@ -0,0 +1,54 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - CSSGroupingRule - cssRules</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssgroupingrule-interface">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script>
+    function create(t) {
+        var style = document.createElement('style');
+        style.appendChild(document.createTextNode('@media all { * {} }'));
+        document.head.appendChild(style);
+        t.add_cleanup(function() {
+            document.head.removeChild(style);
+        });
+
+        assert_true(!!style.sheet, 'setup - sheet defined');
+        assert_equals(
+            style.sheet.cssRules.length, 1, 'setup - grouping rule created'
+        );
+        assert_equals(
+            style.sheet.cssRules[0].cssRules.length, 1, 'setup - rule created'
+        );
+        return style.sheet.cssRules[0];
+    }
+
+    test(function (t) {
+        var groupingRule = create(t);
+        groupingRule.cssRules.wptMarker = 'wpt';
+
+        // `insertRule` is used to prompt non-conforming implementations to
+        // create a new CSSRuleList object. Its behavior is verified by a
+        // dedicated test and should not influence the result of this
+        // particular test.
+        try {
+            groupingRule.insertRule('.foo {}', 0);
+            groupingRule.insertRule('.bar {}', 0);
+            groupingRule.insertRule('.baz {}', 0);
+        } catch (err) {}
+
+        assert_equals(groupingRule.cssRules.wptMarker, 'wpt');
+
+        try {
+            groupingRule.deleteRule('.foo {}', 0);
+            groupingRule.deleteRule('.bar {}', 0);
+            groupingRule.deleteRule('.baz {}', 0);
+        } catch (err) {}
+
+        assert_equals(groupingRule.cssRules.wptMarker, 'wpt');
+    }, '[SameObject] is honored');
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/CSSGroupingRule-insertRule.html b/testing/web-platform/tests/css/cssom/CSSGroupingRule-insertRule.html
new file mode 100644
index 0000000000..d131e3975d
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSGroupingRule-insertRule.html
@@ -0,0 +1,118 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - CSSGroupingRule - insertRule</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssgroupingrule-interface">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script>
+    function create(t) {
+        var style = document.createElement('style');
+        style.appendChild(document.createTextNode('@media all { * {} }'));
+        document.head.appendChild(style);
+        t.add_cleanup(function() {
+            document.head.removeChild(style);
+        });
+
+        assert_true(!!style.sheet, 'setup - sheet defined');
+        assert_equals(
+            style.sheet.cssRules.length, 1, 'setup - grouping rule created'
+        );
+        assert_equals(
+            style.sheet.cssRules[0].cssRules.length, 1, 'setup - rule created'
+        );
+        return style.sheet.cssRules[0];
+    }
+
+    test(function (t) {
+        var groupingRule = create(t);
+        var first = groupingRule.cssRules[0].cssText;
+        var result;
+
+        result = groupingRule.insertRule('.foo {}', 0);
+
+        assert_equals(groupingRule.cssRules.length, 2);
+        assert_not_equals(groupingRule.cssRules[0].cssText, first);
+        assert_equals(groupingRule.cssRules[1].cssText, first);
+        assert_equals(result, 0, 'result');
+    }, 'index before first');
+
+    test(function (t) {
+        var groupingRule = create(t);
+        var first = groupingRule.cssRules[0].cssText;
+        var result;
+
+        result = groupingRule.insertRule('.foo {}', 1);
+
+        assert_equals(groupingRule.cssRules.length, 2);
+        assert_equals(groupingRule.cssRules[0].cssText, first);
+        assert_not_equals(groupingRule.cssRules[1].cssText, first);
+        assert_equals(result, 1);
+    }, 'index after final');
+
+    test(function (t) {
+        var groupingRule = create(t);
+        var first = groupingRule.cssRules[0].cssText;
+        var result;
+
+        result = groupingRule.insertRule('.foo {}');
+
+        assert_equals(groupingRule.cssRules.length, 2);
+        assert_not_equals(groupingRule.cssRules[0].cssText, first);
+        assert_equals(groupingRule.cssRules[1].cssText, first);
+        assert_equals(result, 0);
+    }, 'index not specified');
+
+    test(function (t) {
+        var groupingRule = create(t);
+        var first = groupingRule.cssRules[0].cssText;
+
+        assert_throws_dom('IndexSizeError', function() {
+            // The syntax error is intentional; it verifies that the insertion
+            // index is validated prior to the CSS text.
+            groupingRule.insertRule('???', 2);
+        });
+
+        assert_equals(groupingRule.cssRules.length, 1);
+        assert_equals(groupingRule.cssRules[0].cssText, first);
+    }, 'index exceeds length');
+
+    test(function (t) {
+        var groupingRule = create(t);
+        var first = groupingRule.cssRules[0].cssText;
+
+        assert_throws_dom('SyntaxError', function() {
+            groupingRule.insertRule('???', 0);
+        });
+
+        assert_equals(groupingRule.cssRules.length, 1);
+        assert_equals(groupingRule.cssRules[0].cssText, first);
+    }, 'CSS parsing error');
+
+    test(function (t) {
+        var groupingRule = create(t);
+        var first = groupingRule.cssRules[0].cssText;
+
+        assert_throws_dom('HierarchyRequestError', function() {
+            groupingRule.insertRule('@import url("foo.css");', 0);
+        });
+
+        assert_equals(groupingRule.cssRules.length, 1);
+        assert_equals(groupingRule.cssRules[0].cssText, first);
+    }, 'constraint violation');
+
+    test(function (t) {
+        var groupingRule = create(t);
+        var first = groupingRule.cssRules[0].cssText;
+
+        assert_throws_dom('HierarchyRequestError', function() {
+            groupingRule.insertRule('@namespace url(http://www.w3.org/1999/xhtml);', 0);
+        });
+
+        assert_equals(groupingRule.cssRules.length, 1);
+        assert_equals(groupingRule.cssRules[0].cssText, first);
+    }, 'disallowed namespace rule');
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/CSSKeyframeRule.html b/testing/web-platform/tests/css/cssom/CSSKeyframeRule.html
new file mode 100644
index 0000000000..3f6d182186
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSKeyframeRule.html
@@ -0,0 +1,52 @@
+<!doctype html>
+<meta charset="utf-8">
+<title></title>
+<link rel="help" href="https://drafts.csswg.org/css-animations/#interface-csskeyframerule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style type="text/css" id="styleElement">
+  div { animation: 3s slidein; }
+  @keyframes slidein {
+    from {
+      margin-left: 100%;
+      width: 300%;
+    }
+
+    to {
+      margin-left: 0%;
+      width: 100%;
+    }
+  }
+</style>
+<script>
+  var styleSheet = document.getElementById("styleElement").sheet;
+  var keyframesRule = styleSheet.cssRules[1];
+  var fromRule = keyframesRule.cssRules[0];
+  var toRule = keyframesRule.cssRules[1];
+
+  test(function() {
+    assert_equals(keyframesRule.name, "slidein");
+    assert_equals(typeof fromRule.style, "object");
+    assert_equals(fromRule.style.marginLeft, "100%");
+    assert_equals(fromRule.style.width, "300%");
+
+    assert_equals(typeof toRule.style, "object");
+    assert_equals(toRule.style.marginLeft, "0%");
+    assert_equals(toRule.style.width, "100%");
+
+    toRule.style.marginLeft = "-5%";
+    toRule.style.width = "50%";
+
+    assert_equals(fromRule.style.marginLeft, "100%");
+    assert_equals(fromRule.style.width, "300%");
+    assert_equals(toRule.style.marginLeft, "-5%");
+    assert_equals(toRule.style.width, "50%");
+  }, "CSSKeyframeRule: style property");
+
+  test(function() {
+    fromRule.style = "margin-left: 50%; width: 100%;";
+
+    assert_equals(fromRule.style.marginLeft, "50%", "margin-left");
+    assert_equals(fromRule.style.width, "100%", "width");
+  }, "CSSKeyframeRule: style property has [PutForwards]");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSKeyframesRule.html b/testing/web-platform/tests/css/cssom/CSSKeyframesRule.html
new file mode 100644
index 0000000000..a1adac383f
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSKeyframesRule.html
@@ -0,0 +1,85 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM -  CSSKeyframesRule interface</title>
+    <link rel="help" href="https://drafts.csswg.org/css-animations/#interface-csskeyframesrule">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style>
+        @keyframes foo {
+            0% { top: 0px; }
+            100% { top: 200px; }
+        }
+        @keyframes empty {}
+        @keyframes indexed-access {
+            0% { top: 0px; }
+            100% { top: 200px; }
+        }
+    </style>
+
+    <script>
+    test(function () {
+        var keyframe = document.styleSheets[0].cssRules[0];
+        assert_equals(keyframe.name, "foo", "CSSKeyframesRule name attribute");
+        assert_equals(keyframe.cssRules.length, 2, "CSSKeyframesRule cssRule length attribute");
+        assert_equals(keyframe.cssRules[0].cssText, "0% { top: 0px; }", "CSSKeyframesRule cssRule cssText attribute");
+        assert_equals(keyframe.cssRules[1].cssText, "100% { top: 200px; }", "CSSKeyframesRule cssRule cssText attribute");
+
+        keyframe.appendRule("50% { top: 100px; }");
+        assert_equals(keyframe.cssRules.length, 3, "CSSKeyframesRule cssRule length attribute after appendRule function");
+        assert_equals(keyframe.cssRules[0].cssText, "0% { top: 0px; }", "CSSKeyframesRule cssRule cssText attribute after appendRule function");
+        assert_equals(keyframe.cssRules[1].cssText, "100% { top: 200px; }", "CSSKeyframesRule cssRule cssText attribute after appendRule function");
+        assert_equals(keyframe.cssRules[2].cssText, "50% { top: 100px; }", "CSSKeyframesRule cssRule cssText attribute after appendRule function");
+
+        keyframe.appendRule("0% { top: 50px; }");
+        assert_equals(keyframe.cssRules.length, 4, "CSSKeyframesRule cssRule length attribute after appendRule function");
+        assert_equals(keyframe.cssRules[0].cssText, "0% { top: 0px; }", "CSSKeyframesRule cssRule cssText attribute after appendRule function");
+        assert_equals(keyframe.cssRules[1].cssText, "100% { top: 200px; }", "CSSKeyframesRule cssRule cssText attribute after appendRule function");
+        assert_equals(keyframe.cssRules[2].cssText, "50% { top: 100px; }", "CSSKeyframesRule cssRule cssText attribute after appendRule function");
+        assert_equals(keyframe.cssRules[3].cssText, "0% { top: 50px; }", "CSSKeyframesRule cssRule cssText attribute after appendRule function");
+
+        var find1 = keyframe.findRule("50%");
+        assert_equals(find1.cssText, "50% { top: 100px; }", "CSSKeyframesRule findRule function");
+        var find2 = keyframe.findRule("0%");
+        assert_equals(find2.cssText, "0% { top: 50px; }", "CSSKeyframesRule findRule function");
+        var find3 = keyframe.findRule("70%");
+        assert_equals(find3, null, "CSSKeyframesRule findRule function");
+
+        keyframe.deleteRule("100%");
+        assert_equals(keyframe.cssRules.length, 3, "CSSKeyframesRule cssRule length attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[0].cssText, "0% { top: 0px; }", "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[1].cssText, "50% { top: 100px; }", "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[2].cssText, "0% { top: 50px; }", "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[3], undefined, "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+
+        keyframe.deleteRule("80%");
+        assert_equals(keyframe.cssRules.length, 3, "CSSKeyframesRule cssRule length attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[0].cssText, "0% { top: 0px; }", "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[1].cssText, "50% { top: 100px; }", "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[2].cssText, "0% { top: 50px; }", "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+        assert_equals(keyframe.cssRules[3], undefined, "CSSKeyframesRule cssRule cssText attribute after deleteRule function");
+
+        var empty = document.styleSheets[0].cssRules[1];
+        empty.name = "bar";
+        assert_equals(empty.name, "bar", "CSSKeyframesRule name setter");
+        assert_equals(empty.cssText.replace(/\s/g, ""), "@keyframesbar{}", "CSSKeyframesRule cssText attribute");
+
+        empty.name = "initial";
+        assert_equals(empty.name, "initial", "CSSKeyframesRule name setter, CSS-wide keyword");
+        assert_equals(empty.cssText.replace(/\s/g, ""), "@keyframes\"initial\"{}", "CSSKeyframesRule cssText attribute with CSS-wide keyword name");
+
+        empty.name = "none";
+        assert_equals(empty.name, "none", "CSSKeyframesRule name setter, 'none'");
+        assert_equals(empty.cssText.replace(/\s/g, ""), "@keyframes\"none\"{}", "CSSKeyframesRule cssText attribute with 'none' name");
+    }, 'name, cssRules, appendRule, findRule, deleteRule');
+
+    test(function () {
+        const keyframes = document.styleSheets[0].cssRules[2];
+        assert_equals(keyframes[0].cssText, "0% { top: 0px; }", "CSSKeyframesRule indexed getter [0]");
+        assert_equals(keyframes[1].cssText, "100% { top: 200px; }", "CSSKeyframesRule indexed getter [1]");
+        assert_equals(keyframes.length, 2, "CSSKeyframesRule.length");
+    }, 'indexed getter, length');
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/CSSNamespaceRule.html b/testing/web-platform/tests/css/cssom/CSSNamespaceRule.html
new file mode 100644
index 0000000000..fbbaa3c569
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSNamespaceRule.html
@@ -0,0 +1,32 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - CSSNamespaceRule interface</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssnamespacerule-interface">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style>
+        @namespace svg url(http://servo);
+        @namespace url(http://servo1);
+        @namespace svg url("http://servo2");
+    </style>
+
+    <script>
+    test(function () {
+        var rules = document.styleSheets[0].cssRules;
+        assert_equals(rules[0].prefix, "svg", "CSSNamespaceRule prefix attribute");
+        assert_equals(rules[0].namespaceURI, "http://servo", "CSSNamespaceRule namespaceURI attribute");
+        assert_equals(rules[0].cssText, "@namespace svg url(\"http://servo\");", "CSSNamespaceRule cssText attribute");
+
+        assert_equals(rules[1].prefix, "", "CSSNamespaceRule prefix attribute");
+        assert_equals(rules[1].namespaceURI, "http://servo1", "CSSNamespaceRule namespaceURI attribute");
+        assert_equals(rules[1].cssText, "@namespace url(\"http://servo1\");", "CSSNamespaceRule cssText attribute");
+
+        assert_equals(rules[2].prefix, "svg", "CSSNamespaceRule prefix attribute");
+        assert_equals(rules[2].namespaceURI, "http://servo2", "CSSNamespaceRule namespaceURI attribute");
+        assert_equals(rules[2].cssText, "@namespace svg url(\"http://servo2\");", "CSSNamespaceRule cssText attribute");
+    });
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/CSSRuleList.html b/testing/web-platform/tests/css/cssom/CSSRuleList.html
new file mode 100644
index 0000000000..4605b74964
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSRuleList.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - CSSRuleList interface</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssrulelist-interface">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style>
+        body { width: 50%; }
+        #foo { height: 100px; }
+    </style>
+
+    <script>
+    test(function () {
+        var ruleList = document.styleSheets[0].cssRules;
+        assert_equals(ruleList.length, 2, "CSSRuleList length attribute");
+        assert_equals(ruleList[0].cssText, "body { width: 50%; }", "CSSRuleList indexed getter");
+        assert_equals(ruleList[1].cssText, "#foo { height: 100px; }", "CSSRuleList indexed getter");
+        assert_equals(ruleList[2], undefined, "CSSRuleList indexed getter");
+        assert_equals(ruleList.item(0).cssText, "body { width: 50%; }", "CSSRuleList item function");
+        assert_equals(ruleList.item(1).cssText, "#foo { height: 100px; }", "CSSRuleList item function");
+        assert_equals(ruleList.item(2), null, "CSSRuleList item function");
+    });
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText-namespace.html b/testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText-namespace.html
new file mode 100644
index 0000000000..4da0a333e9
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText-namespace.html
@@ -0,0 +1,60 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>CSSOM StyleRule selectorText property setter with namespaces</title>
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#dom-cssstylerule-selectortext">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+
+<style type="text/css" id="styleElement">
+@namespace url(http://www.w3.org/1999/xhtml);
+@namespace svg url(http://www.w3.org/2000/svg);
+
+svg|*.style0 { background-color: rgb(0, 0, 255) !important; }
+svg|*.style1 { background-color: rgb(255, 0, 255); }
+</style>
+
+<span>
+  <p></p>
+
+  <svg height="30" width="200" id="container" class="style1" lang="zh-CN" language segment="42 43">
+    <text x="0" y="15">SVG text</text>
+  </svg>
+</span>
+
+<script>
+  var styleSheet = document.getElementById("styleElement").sheet;
+  var rule = styleSheet.cssRules[2];
+
+  var divContainerStyle = getComputedStyle(document.getElementById("container"));
+
+  const originalStyleSelector = "svg|*.style0";
+
+  var assertColors = function(selectorMatches) {
+    assert_equals(divContainerStyle.getPropertyValue('background-color'), selectorMatches ? "rgb(0, 0, 255)" : "rgb(255, 0, 255)")
+  };
+
+  [
+    {selector: ".style1", isMatch: false, },
+    {selector: "svg|*.style1  ", isMatch: true, normalizedSelector: "svg|*.style1"},
+    {selector: "*|*.style1  ", isMatch: true, normalizedSelector: "*|*.style1"},
+    {selector: " *.style1  ", isMatch: false, normalizedSelector: ".style1"},
+    {selector: "p", isMatch: false},
+  ].forEach(function(testCase) {
+    test(function() {
+      // Check if starting with the default value.
+      assert_equals(rule.selectorText, originalStyleSelector);
+
+      this.add_cleanup(function() { rule.selectorText = originalStyleSelector; });
+
+      assertColors(false);
+
+      rule.selectorText = testCase.selector;
+
+      var expectedSelector = testCase.normalizedSelector ? testCase.normalizedSelector : testCase.selector;
+
+      assert_equals(rule.selectorText, expectedSelector);
+
+      assertColors(testCase.isMatch);
+    }, "CSSStyleRule: selectorText value: |" + testCase.selector + "| isMatch: " + testCase.isMatch);
+  });
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText.html b/testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText.html
new file mode 100644
index 0000000000..e29db52ec6
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleRule-set-selectorText.html
@@ -0,0 +1,158 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>CSSOM StyleRule selectorText property setter</title>
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#dom-cssstylerule-selectortext">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+
+<style type="text/css" id="styleElement">
+  .style0 { background-color: rgb(0, 0, 255) !important; }
+  .style1 { background-color: rgb(255, 0, 255); }
+</style>
+
+<span>
+  <p></p>
+  <div id="container" class="style1" lang="zh-CN" language segment="42 43">
+  </div>
+</span>
+
+<script>
+  var styleSheet = document.getElementById("styleElement").sheet;
+  var rule = styleSheet.cssRules[0];
+
+  var divContainerStyle = getComputedStyle(document.getElementById("container"));
+
+  const originalStyleSelector = ".style0";
+
+  var assertColors = function(selectorMatches) {
+    assert_equals(divContainerStyle.backgroundColor, selectorMatches ? "rgb(0, 0, 255)" : "rgb(255, 0, 255)")
+  };
+
+  test(function() {
+    assert_equals(typeof rule.selectorText, "string");
+    assert_equals(rule.selectorText, originalStyleSelector);
+  }, "CSSStyleRule: Can read selectorText value.");
+
+  [ // Invalid selector values.
+    "",
+    " ",
+    "!!",
+    "123",
+    "-",
+    "$",
+    ":",
+    "::",
+    ":::",
+    "::gibberish",
+    ":gibberish",
+    ".",
+    "#",
+    "[]",
+    "[",
+    "()",
+    "(",
+    "{}",
+    "{",
+  ].forEach(function(selector) {
+    test(function() {
+      assert_equals(rule.selectorText, originalStyleSelector);
+
+      this.add_cleanup(function() { rule.selectorText = originalStyleSelector; });
+
+      rule.selectorText = selector;
+
+      assert_equals(rule.selectorText, originalStyleSelector);
+    }, "CSSStyleRule: Invalid CSS selector: " + selector);
+  });
+
+
+  [ // Valid selector values.
+    {selector: "#container", isMatch: true},
+    {selector: "#container  ", isMatch: true, normalizedSelector: "#container"},
+    {selector: "  #container ", isMatch: true, normalizedSelector: "#container"},
+    {selector: ".style1", isMatch: true},
+    {selector: "div.style1", isMatch: true},
+    {selector: "div:not(#non-existing-id)", isMatch: true},
+    {selector: "div", isMatch: true},
+    {selector: "*", isMatch: true},
+
+    {selector: "#no-match", isMatch: false},
+    {selector: "ÇĞıİ", isMatch: false},
+    {selector: "🤓", isMatch: false},
+
+    {selector: "[language]", isMatch: true},
+    {selector: "[language-no]", isMatch: false},
+    {selector: "[lang=\"zh-CN\"]", isMatch: true},
+    {selector: "[lang=\"ab-CD\"]", isMatch: false},
+    {selector: "[segment~=\"43\"]", isMatch: true},
+    {selector: "[segment~=\"42\"]", isMatch: true},
+    {selector: "[lang|=\"zh\"]", isMatch: true},
+    {selector: "[lang|=\"zh-CN\"]", isMatch: true},
+    {selector: "[lang|=\"ab\"]", isMatch: false},
+    {selector: "[lang|=\"z\"]", isMatch: false},
+    {selector: "[lang^=\"z\"]", isMatch: true},
+    {selector: "[lang^=\"ab\"]", isMatch: false},
+    {selector: "[segment$=\"43\"]", isMatch: true},
+    {selector: "[segment$=\"3\"]", isMatch: true},
+    {selector: "[segment$=\"42\"]", isMatch: false},
+    {selector: "[lang*=\"-\"]", isMatch: true},
+    {selector: "[lang*=\"h-\"]", isMatch: true},
+    {selector: "[lang*=\"ab\"]", isMatch: false},
+
+    {selector: "*|div", isMatch: true, normalizedSelector: "div"},
+    {selector: "|div", isMatch: false},
+    {selector: "*|a", isMatch: false, normalizedSelector: "a"},
+    {selector: "*|*", isMatch: true, normalizedSelector: "*"},
+    {selector: "[*|lang]", isMatch: true, normalizedSelector: "[*|lang]"},
+    {selector: "[|lang]", isMatch: true, normalizedSelector: "[lang]"},
+
+    {selector: ":active", isMatch: false},
+    {selector: ":not(:active)", isMatch: true},
+    {selector: "*:not(:active)", isMatch: true, normalizedSelector: ":not(:active)"},
+    {selector: "div:not(:active)", isMatch: true},
+    {selector: "div:active", isMatch: false},
+
+    {selector: "span div", isMatch: true},
+    {selector: "span  div  ", isMatch: true, normalizedSelector: "span div"},
+    {selector: "span > div", isMatch: true},
+    {selector: "div div", isMatch: false},
+    {selector: "div > div", isMatch: false},
+    {selector: "p + div", isMatch: true},
+    {selector: "span + div", isMatch: false},
+    {selector: "p ~ div", isMatch: true},
+    {selector: "span ~ div", isMatch: false},
+
+    {selector: ":lang(zh-CN)", isMatch: true},
+    {selector: ":lang(zh)", isMatch: true},
+    {selector: ":lang(tr-AZ)", isMatch: false},
+
+    {selector: "::after", isMatch: false, normalizedSelector: "::after"},
+    {selector: ":after", isMatch: false, normalizedSelector: "::after"},
+    {selector: "::before", isMatch: false, normalizedSelector: "::before"},
+    {selector: ":before", isMatch: false, normalizedSelector: "::before"},
+    {selector: "::first-letter", isMatch: false, normalizedSelector: "::first-letter"},
+    {selector: ":first-letter", isMatch: false, normalizedSelector: "::first-letter"},
+    {selector: "::first-line", isMatch: false, normalizedSelector: "::first-line"},
+    {selector: ":first-line", isMatch: false, normalizedSelector: "::first-line"},
+
+    {selector: "div:focus:not([lang=\"zh-CN\"])", isMatch: false},
+    {selector: "div[lang=\"zh-CN\"]:not(:focus)", isMatch: true},
+  ].forEach(function(testCase) {
+    test(function() {
+      // Check if starting with the default value.
+      assert_equals(rule.selectorText, originalStyleSelector);
+
+      this.add_cleanup(function() { rule.selectorText = originalStyleSelector; });
+
+      assertColors(false);
+
+      rule.selectorText = testCase.selector;
+
+      var expectedSelector = testCase.normalizedSelector ? testCase.normalizedSelector : testCase.selector;
+
+      assert_equals(rule.selectorText, expectedSelector);
+
+      assertColors(testCase.isMatch);
+    }, "CSSStyleRule: selectorText value: |" + testCase.selector + "| isMatch: " + testCase.isMatch);
+  });
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleRule.html b/testing/web-platform/tests/css/cssom/CSSStyleRule.html
new file mode 100644
index 0000000000..3acdfb1285
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleRule.html
@@ -0,0 +1,113 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSSOM CSSRule CSSStyleRule interface</title>
+    <link rel="author" title="Letitia Lew" href="mailto:lew.letitia@gmail.com">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#css-rules">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstylerule-interface">
+    <meta name="flags" content="dom">
+    <meta name="assert" content="All properties for this CSSStyleRule instance of CSSRule are initialized correctly">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+
+    <style id="styleElement" type="text/css">
+        div { margin: 10px; padding: 0px; }
+    </style>
+</head>
+<body>
+    <div id="log"></div>
+
+    <script type="text/javascript">
+        var rule;
+        setup(function() {
+            var styleSheet = document.getElementById("styleElement").sheet;
+            var ruleList = styleSheet.cssRules;
+            rule = ruleList[0];
+        });
+
+        test(function() {
+            assert_true(rule instanceof CSSRule);
+            assert_true(rule instanceof CSSStyleRule);
+        }, "CSSRule and CSSStyleRule types");
+
+        test(function() {
+            assert_equals(rule.STYLE_RULE, 1);
+            assert_equals(rule.IMPORT_RULE, 3);
+            assert_equals(rule.MEDIA_RULE, 4);
+            assert_equals(rule.FONT_FACE_RULE, 5);
+            assert_equals(rule.PAGE_RULE, 6);
+            assert_equals(rule.NAMESPACE_RULE, 10);
+            assert_idl_attribute(rule, "type");
+            assert_equals(typeof rule.type, "number");
+        }, "Type of CSSRule#type and constant values");
+
+        test(function() {
+            assert_true(rule instanceof CSSRule);
+            assert_idl_attribute(rule, "cssText");
+            assert_idl_attribute(rule, "parentRule");
+            assert_idl_attribute(rule, "parentStyleSheet");
+        }, "Existence of CSSRule attributes");
+
+        test(function() {
+            assert_readonly(rule, "type");
+            assert_readonly(rule, "parentRule");
+            assert_readonly(rule, "parentStyleSheet");
+        }, "Writability of CSSRule attributes");
+
+        test(function() {
+            assert_equals(rule.type, rule.STYLE_RULE);
+            assert_equals(typeof rule.cssText, "string");
+            assert_equals(rule.cssText, "div { margin: 10px; padding: 0px; }");
+            assert_equals(rule.parentRule, null);
+            assert_true(rule.parentStyleSheet instanceof CSSStyleSheet);
+        }, "Values of CSSRule attributes");
+
+        test(function() {
+            assert_idl_attribute(rule, "selectorText");
+            assert_equals(typeof rule.selectorText, "string");
+            assert_idl_attribute(rule, "style");
+        }, "Existence and type of CSSStyleRule attributes");
+
+        test(function() {
+            // CSSStyleRule.style has PutForwards=cssText and SameObject.
+            var initial = rule.style.cssText;
+            var style = rule.style;
+
+            rule.style = "";
+            assert_equals(rule.style.cssText, "");
+            assert_equals(rule.style, style);
+
+            rule.style = "margin: 42px;";
+            assert_equals(rule.style.margin, "42px");
+            assert_equals(rule.style, style);
+
+            rule.style = initial;
+            assert_equals(rule.style, style);
+        }, "Assigning to CSSStyleRule.style assigns to cssText; CSSStyleRule.style returns the same object");
+
+        test(function() {
+            assert_equals(rule.selectorText, "div");
+            assert_true(rule.style instanceof CSSStyleDeclaration);
+        }, "Values of CSSStyleRule attributes");
+
+        test(function() {
+            assert_equals(rule.style.margin, "10px");
+            assert_equals(rule.style.padding, "0px");
+
+            rule.style.padding = "5px";
+            rule.style.border = "1px solid";
+
+            assert_equals(rule.style.padding, "5px");
+            assert_equals(rule.style.border, "1px solid");
+        }, "Mutability of CSSStyleRule's style attribute");
+
+        test(function() {
+            rule.style = "margin: 15px; padding: 2px;";
+
+            assert_equals(rule.style.margin, "15px", "margin");
+            assert_equals(rule.style.padding, "2px", "padding");
+        }, "CSSStyleRule's style has [PutForwards]");
+    </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-baseURL.tentative.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-baseURL.tentative.html
new file mode 100644
index 0000000000..8997a59e9c
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-baseURL.tentative.html
@@ -0,0 +1,66 @@
+<!DOCTYPE html>
+<title>CSSStyleSheet baseURL</title>
+<link rel="author" title="Erik Nordin" href="mailto:enordin@mozilla.com">
+<link rel="help" href="https://github.com/WICG/construct-stylesheets/issues/95#issuecomment-593545252">
+<div id="target"></div>
+<script src='/resources/testharness.js'></script>
+<script src='/resources/testharnessreport.js'></script>
+<script>
+
+function currentLocation() {
+  const sections = location.href.split("/")
+  sections.pop();
+  return sections.join("/");
+}
+
+test(() => {
+  const span = document.createElement("span");
+  target.appendChild(span);
+  span.attachShadow({ mode: "open" })
+  const shadowDiv = document.createElement("div");
+  span.shadowRoot.appendChild(shadowDiv);
+
+  const fileName = "example.png"
+  const baseURL = `${location.origin}/custom/path/`;
+  const fullURL = `${baseURL}${fileName}`;
+
+  const sheet = new CSSStyleSheet({ baseURL });
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+
+  sheet.replaceSync(`* { background-image: url("${fileName}"); }`);
+  const styleFromRelative = getComputedStyle(shadowDiv).backgroundImage;
+
+  sheet.replaceSync(`* { background-image: url("${fullURL}"); }`);
+  const styleFromFull = getComputedStyle(shadowDiv).backgroundImage;
+
+  assert_equals(styleFromRelative, styleFromFull);
+}, "Constructing sheet with custom base URL ueses that URL for CSS rules");
+
+test(() => {
+  const span = document.createElement("span");
+  target.appendChild(span);
+  span.attachShadow({ mode: "open" })
+  const shadowDiv = document.createElement("div");
+  span.shadowRoot.appendChild(shadowDiv);
+
+  const fileName = "example.png"
+  const baseURL = "custom/path/";
+  const fullURL = `${currentLocation()}/${baseURL}${fileName}`;
+
+  const sheet = new CSSStyleSheet({ baseURL });
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+
+  sheet.replaceSync(`* { background-image: url("${fileName}"); }`);
+  const styleFromRelative = getComputedStyle(shadowDiv).backgroundImage;
+
+  sheet.replaceSync(`* { background-image: url("${fullURL}"); }`);
+  const styleFromFull = getComputedStyle(shadowDiv).backgroundImage;
+
+  assert_equals(styleFromRelative, styleFromFull);
+}, "Constructing sheet with relative URL adds to the constructor document's base URL");
+
+test(() => {
+  assert_throws_dom("NotAllowedError", () => { new CSSStyleSheet({ baseURL: "https://test:test/"}) });
+}, "Constructing sheet with invalid base URL throws a NotAllowedError");
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat-ref.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat-ref.html
new file mode 100644
index 0000000000..1c6b6784c9
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Adoptedstylesheets.concat should work when starting empty</title>
+<link rel="author" href="mailto:masonf@chromium.org">
+
+<span>This should be green</span><br>
+<span>This should be green</span>
+<style>
+  span {
+    background-color:green;
+  }
+</style>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat.html
new file mode 100644
index 0000000000..3ba9992845
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-concat.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Adoptedstylesheets.concat should work when starting empty</title>
+<link rel="author" href="mailto:masonf@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
+<link rel="match" href="CSSStyleSheet-constructable-concat-ref.html">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<span>This should be green</span>
+<div id=host></div>
+
+<script>
+test(function() {
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync('span {background-color:green;}');
+  assert_equals(document.adoptedStyleSheets.length,0);
+  document.adoptedStyleSheets = document.adoptedStyleSheets.concat([sheet]);
+  assert_equals(document.adoptedStyleSheets.length,1);
+
+  const host = document.getElementById('host');
+  const shadow = host.attachShadow({mode: 'open'});
+  shadow.innerHTML = '<span>This should be green</span>';
+  assert_equals(shadow.adoptedStyleSheets.length,0);
+  shadow.adoptedStyleSheets = shadow.adoptedStyleSheets.concat([sheet]);
+  assert_equals(shadow.adoptedStyleSheets.length,1);
+}, "adoptedStyleSheets should allow .concat on empty starting values");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-cssRules.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-cssRules.html
new file mode 100644
index 0000000000..72b0e15c8b
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-cssRules.html
@@ -0,0 +1,22 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSStyleSheet.replace/replaceSync() doesn't change cssRules object</title>
+<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel="author" href="https://mozilla.org" title="Mozilla">
+<link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1752392">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<span>Should be green</span>
+<script>
+promise_test(async function() {
+  const sheet = new CSSStyleSheet();
+  let rules = sheet.cssRules;
+  sheet.replaceSync('span {color:blue;}');
+  assert_equals(rules, sheet.cssRules, "Rules should be the same after replaceSync");
+  await sheet.replace('span {color: lime;}');
+  assert_equals(rules, sheet.cssRules, "Rules should be the same after replace()");
+  document.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(document.querySelector("span")).color, "rgb(0, 255, 0)", "Sheet should apply");
+}, "cssRules doesn't change on replace / replaceSync");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disabled-regular-sheet-insertion.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disabled-regular-sheet-insertion.html
new file mode 100644
index 0000000000..ef4ea14806
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disabled-regular-sheet-insertion.html
@@ -0,0 +1,22 @@
+<!doctype html>
+<title>Shouldn't crash / assert when inserting a stylesheet after there are disabled constructable sheets</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://wicg.github.io/construct-stylesheets/">
+<script src = '/resources/testharness.js'></script>
+<script src = '/resources/testharnessreport.js'></script>
+<div id="host"></div>
+<script>
+test(function() {
+  let sheet = new CSSStyleSheet({ disabled: true });
+  sheet.replaceSync("div { color: red }");
+
+  let root = document.getElementById("host").attachShadow({ mode: "open" });
+  root.adoptedStyleSheets = [sheet];
+  let style = document.createElement("style");
+  root.innerHTML = `
+    <style>div { color: green }</style>
+    <div>Should be green</div>
+  `;
+  assert_equals(getComputedStyle(root.querySelector("div")).color, "rgb(0, 128, 0)", "Should insert the sheet at the right position and not crash");
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disallow-import.tentative.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disallow-import.tentative.html
new file mode 100644
index 0000000000..d49a084fbe
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-disallow-import.tentative.html
@@ -0,0 +1,83 @@
+<!DOCTYPE html>
+<title>CSSStyleSheet Disallow Import Rules</title>
+<link rel="author" title="Erik Nordin" href="mailto:enordin@mozilla.com">
+<link rel="help" href="https://github.com/WICG/construct-stylesheets/issues/119#issuecomment-597733392">
+<div id="target"></div>
+<script src='/resources/testharness.js'></script>
+<script src='/resources/testharnessreport.js'></script>
+<script>
+
+const greenStyleText = ".green { color: green; }";
+const import_text = '@import url("support/constructable-import.css");';
+
+function attachShadowDiv(host) {
+  const shadowRoot = host.attachShadow({mode: 'open'});
+  const shadowDiv = document.createElement("div");
+  shadowRoot.appendChild(shadowDiv);
+  return shadowDiv;
+}
+
+test(() => {
+  assert_throws_dom("SyntaxError", () => { (new CSSStyleSheet).insertRule(import_text) });
+}, 'Inserting an @import rule through insertRule on a constructed stylesheet throws an exception');
+
+promise_test(async t => {
+    const importUrl = "support/constructable-import.css";
+    const sheet = new CSSStyleSheet();
+
+    sheet.replaceSync(import_text);
+    await sheet.replace(import_text);
+    assert_throws_dom("SyntaxError", () => { sheet.insertRule(import_text) });
+
+    const timeAfterImportsInvoked = performance.now();
+
+    let link = document.createElement("link");
+    t.add_cleanup(() => { link.remove(); });
+    link.rel = "stylesheet";
+    link.href = importUrl;
+
+    await new Promise((resolve, reject) => {
+      link.addEventListener("load", resolve);
+      link.addEventListener("error", reject);
+      document.body.appendChild(link);
+    });
+
+    let entries = window.performance.getEntriesByType('resource').filter(entry => entry.name.includes(importUrl));
+    assert_equals(entries.length, 1, "There should be only one entry for the import URL");
+    assert_greater_than_equal(entries[0].startTime, timeAfterImportsInvoked, "The entry's start time should be after all throws");
+}, "@import rules should not trigger any loads.")
+
+promise_test(() => {
+  const span = document.createElement("span");
+  target.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  shadowDiv.classList.add("green");
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  const sheet_promise = sheet.replace(`${import_text} ${greenStyleText}`);
+  return sheet_promise.then((sheet) => {
+    assert_equals(sheet.cssRules.length, 1);
+    assert_equals(sheet.cssRules[0].cssText, greenStyleText);
+    assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 128, 0)");
+  }).catch((reason) => {
+    assert_unreached(`Promise was rejected (${reason}) when it should have been resolved`);
+  });
+}, '@import rules are not parsed in CSSStyleSheet.replace');
+
+test(() => {
+  const span = document.createElement("span");
+  target.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  shadowDiv.classList.add("green");
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  // Replace and assert that the imported rule is applied.
+  const sheet_promise = sheet.replaceSync(`${import_text} ${greenStyleText}`);
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(sheet.cssRules[0].cssText, greenStyleText);
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 128, 0)");
+}, '@import rules are not parsed in CSSStyleSheet.replaceSync');
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-duplicate.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-duplicate.html
new file mode 100644
index 0000000000..579f5d0fce
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-duplicate.html
@@ -0,0 +1,84 @@
+<!doctype html>
+<title>Cascade order of a stylesheet for duplicate sheets.</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://wicg.github.io/construct-stylesheets/">
+<script src = '/resources/testharness.js'></script>
+<script src = '/resources/testharnessreport.js'></script>
+<div id="target"></div>
+<script>
+
+function attachShadowDiv(host) {
+  const shadowRoot = host.attachShadow({mode: 'open'});
+  const shadowDiv = document.createElement("div");
+  shadowRoot.appendChild(shadowDiv);
+  return shadowDiv;
+}
+
+function blueSheetsWithIncreasingZIndex(n) {
+  let sheets = [];
+  for (let i = 0; i < n; ++i) {
+    sheets.push(new CSSStyleSheet());
+    sheets[i].replaceSync("div { z-index: " + i + "; color: blue; }");
+  }
+  return sheets;
+}
+
+let sheets = [];
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1]];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+
+  document.adoptedStyleSheets = [];
+}, "Duplicate stylesheets have the right cascade position in the Document");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("div { color: red; }");
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0]];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "0", "backmost stylesheet should take precedence");
+  assert_equals(getComputedStyle(document.querySelector("div")).color, "rgb(0, 0, 255)", "backmost stylesheet should take precedence");
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1], sheet];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "1", "duplicate stylesheet should take the right position in the cascade");
+  assert_equals(getComputedStyle(document.querySelector("div")).color, "rgb(255, 0, 0)", "backmost stylesheet should take precedence");
+
+  document.adoptedStyleSheets = [];
+}, "Appending duplicate stylesheets yields the correct cascade position in the Document");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const span = document.createElement("span");
+  target.appendChild(span);
+  attachShadowDiv(span);
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1]];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+}, "Duplicate stylesheets have the right cascade position in the ShadowRoot");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("div { color: red; }");
+
+  const span = document.createElement("span");
+  target.appendChild(span);
+  attachShadowDiv(span);
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0]];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "0", "backmost stylesheet should take precedence");
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).color, "rgb(0, 0, 255)", "backmost stylesheet should take precedence");
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1], sheet];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).color, "rgb(255, 0, 0)", "backmost stylesheet should take precedence");
+}, "Appending duplicate stylesheets yields the correct cascade position in the ShadowRoot");
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-replace-on-regular-sheet.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-replace-on-regular-sheet.html
new file mode 100644
index 0000000000..76bee452a6
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-replace-on-regular-sheet.html
@@ -0,0 +1,42 @@
+<!doctype html>
+<title>replace / replaceSync on non-constructed stylesheet</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://wicg.github.io/construct-stylesheets/">
+<script src = '/resources/testharness.js'></script>
+<script src = '/resources/testharnessreport.js'></script>
+<style id="style">
+  @import url("support/constructable-import.css");
+  :root { background-color: lime }
+</style>
+<script>
+
+promise_test(async () => {
+  await new Promise(resolve => window.addEventListener("load", resolve));
+  assert_equals(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 255, 0)", "precondition")
+  let sheet = document.styleSheets[0];
+  let childSheet = sheet.cssRules[0].styleSheet;
+  assert_throws_dom("NotAllowedError", () => sheet.replaceSync(":root { background-color: red }"), "replaceSync on non-constructed sheet should throw");
+  assert_throws_dom("NotAllowedError", () => childSheet.replaceSync(":root { background-color: red }"), "replaceSync on non-constructed sheet should throw");
+  assert_equals(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 255, 0)", "old sheet should still apply after replace");
+}, "CSSStyleSheet.replaceSync throws NotAllowedError for non-constructed sheets")
+
+promise_test(async function(t) {
+  assert_equals(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 255, 0)", "precondition")
+  let sheet = document.styleSheets[0];
+  let childSheet = sheet.cssRules[0].styleSheet;
+  await promise_rejects_dom(t, "NotAllowedError", sheet.replace(":root { background-color: red }"), "replace on non-constructed sheet should return a rejected promise");
+  await promise_rejects_dom(t, "NotAllowedError", childSheet.replace(":root { background-color: red }"), "replace on non-constructed sheet should return a rejected promise");
+  assert_equals(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 255, 0)", "old sheet should still apply after replace");
+}, "CSSStyleSheet.replace returns a rejected promise for non-constructed sheets")
+
+promise_test(async function(t) {
+  assert_equals(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 255, 0)", "precondition")
+  let sheet = document.styleSheets[0];
+  let childSheet = sheet.cssRules[0].styleSheet;
+  style.remove() // sheet's associated document becomes null.
+  await promise_rejects_dom(t, "NotAllowedError", sheet.replace(":root { background-color: red }"), "replace on non-constructed sheet should return a rejected promise");
+  await promise_rejects_dom(t, "NotAllowedError", childSheet.replace(":root { background-color: red }"), "replace on non-constructed sheet should return a rejected promise");
+  assert_equals(getComputedStyle(document.documentElement).backgroundColor, "rgba(0, 0, 0, 0)", "old sheet was removed, so the default color should apply");
+}, "CSSStyleSheet.replace returns a rejected promise for non-constructed sheets that have no associated document")
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable.html
new file mode 100644
index 0000000000..f84e4ea9af
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable.html
@@ -0,0 +1,801 @@
+<!DOCTYPE html>
+<title>CSSStyleSheet constructor and adoptedStyleSheets</title>
+<link rel="author" title="Rakina Zata Amni" href="mailto:rakina@chromium.org">
+<link rel="help" href="https://wicg.github.io/construct-stylesheets/">
+<script src = '/resources/testharness.js'></script>
+<script src = '/resources/testharnessreport.js'></script>
+
+<section id="firstSection">
+  <div>
+    <span class="green"></span>
+    <span class="red"></span>
+    <span class="blue"></span>
+    <span class="white"></span>
+    <span class="yellow"></span>
+  </div>
+</section>
+<section id="shadowHost"></section>
+<section id="thirdSection"></section>
+
+<script>
+'use strict';
+const greenStyleText = ".green { color: green; }";
+const redStyleTexts = [".red { color: red; }", ".red + span + span { color: red; }"];
+const blueStyleTexts = [".blue { color: blue; }", ".blue + span + span { color: blue; }"];
+const whiteStyleText = "* { color: white; }";
+const yellowStyleText = ".yellow { color: yellow; }";
+
+const firstDiv = document.querySelector('#firstSection > div');
+const secondDiv = firstDiv.cloneNode(true);
+const shadowHost = document.querySelector('#shadowHost');
+const shadowRoot = shadowHost.attachShadow({mode: 'open'});
+shadowRoot.appendChild(secondDiv);
+
+const greenSpan = firstDiv.children[0];
+const redSpan = firstDiv.children[1];
+const blueSpan = firstDiv.children[2];
+const whiteSpan = firstDiv.children[3];
+const yellowSpan = firstDiv.children[4];
+const greenShadowSpan = secondDiv.children[0];
+const redShadowSpan = secondDiv.children[1];
+const blueShadowSpan = secondDiv.children[2];
+const whiteShadowSpan = secondDiv.children[3];
+const yellowShadowSpan = secondDiv.children[4];
+
+test(() => {
+  assert_equals(document.adoptedStyleSheets.length, 0);
+}, "document.adoptedStyleSheets should initially have length 0.");
+
+test(() => {
+  const sheet = new CSSStyleSheet({disabled: true, media: "screen, print"});
+  assert_equals(sheet.title, null, "The title attribute must return the title or null if title is the empty string");
+  assert_equals(sheet.ownerNode, null);
+  assert_equals(sheet.ownerRule, null);
+  assert_equals(sheet.media.length, 2);
+  assert_equals(sheet.media.item(0), "screen");
+  assert_equals(sheet.media.item(1), "print");
+  assert_true(sheet.disabled);
+  assert_equals(sheet.cssRules.length, 0);
+
+  sheet.insertRule(redStyleTexts[0]);
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(sheet.cssRules[0].cssText, redStyleTexts[0]);
+
+  sheet.insertRule(redStyleTexts[1]);
+  assert_equals(sheet.cssRules.length, 2);
+  assert_equals(sheet.cssRules[0].cssText, redStyleTexts[1]);
+
+  const sheet2 = new CSSStyleSheet({});
+  assert_equals(sheet2.title, null, "The title attribute must return the title or null if title is the empty string");
+  assert_equals(sheet2.ownerNode, null);
+  assert_equals(sheet2.ownerRule, null);
+  assert_equals(sheet2.media.length, 0);
+  assert_false(sheet2.disabled);
+  assert_equals(sheet2.cssRules.length, 0);
+
+  sheet2.insertRule(redStyleTexts[1]);
+  assert_equals(sheet2.cssRules.length, 1);
+  assert_equals(sheet2.cssRules[0].cssText, redStyleTexts[1]);
+
+  sheet2.deleteRule(0);
+  assert_equals(sheet2.cssRules.length, 0);
+
+  const sheet3 = new CSSStyleSheet();
+  assert_equals(sheet3.title, null, "The title attribute must return the title or null if title is the empty string");
+  assert_equals(sheet3.ownerNode, null);
+  assert_equals(sheet3.ownerRule, null);
+  assert_equals(sheet3.media.length, 0);
+  assert_false(sheet3.disabled);
+  assert_equals(sheet3.cssRules.length, 0);
+
+  sheet3.insertRule(redStyleTexts[1]);
+  assert_equals(sheet3.cssRules.length, 1);
+  assert_equals(sheet3.cssRules[0].cssText, redStyleTexts[1]);
+
+  sheet3.deleteRule(0);
+  assert_equals(sheet3.cssRules.length, 0);
+}, 'new CSSStyleSheet produces empty CSSStyleSheet');
+
+test(() => {
+  const sheet = new CSSStyleSheet({title: "something"});
+  assert_equals(sheet.title, null, "title and alternate are not supported by the constructor. https://github.com/WICG/construct-stylesheets/issues/105");
+}, "title can be set in the CSSStyleSheet constructor");
+
+promise_test(() => {
+  const sheet = new CSSStyleSheet({disabled: true, media: "screen, print"});
+  const promise_sheet = sheet.replace(redStyleTexts[0]);
+  return promise_sheet.then(function(sheet) {
+    assert_equals(sheet.title, null, "The title attribute must return the title or null if title is the empty string");
+    assert_equals(sheet.ownerNode, null);
+    assert_equals(sheet.ownerRule, null);
+    assert_equals(sheet.media.length, 2);
+    assert_equals(sheet.media.item(0), "screen");
+    assert_equals(sheet.media.item(1), "print");
+    assert_true(sheet.disabled);
+    assert_equals(sheet.cssRules.length, 1);
+    assert_equals(sheet.cssRules[0].cssText, redStyleTexts[0]);
+
+    sheet.insertRule(redStyleTexts[1]);
+    assert_equals(sheet.cssRules.length, 2);
+    assert_equals(sheet.cssRules[0].cssText, redStyleTexts[1]);
+  });
+}, 'CSSStyleSheet.replace produces Promise<CSSStyleSheet>');
+
+function createAllSheetsPromise() {
+  const greenSheet = new CSSStyleSheet();
+  const redSheet = new CSSStyleSheet({media: "screen, print"});
+  const blueSheet = new CSSStyleSheet({disabled: true});
+  const whiteSheet = new CSSStyleSheet({disabled: true});
+  const yellowSheet = new CSSStyleSheet({disabled: false});
+
+  const greenPromise = greenSheet.replace(greenStyleText);
+  const redPromise = redSheet.replace(redStyleTexts[0] + redStyleTexts[1]);
+  const bluePromise = blueSheet.replace(blueStyleTexts[0] + blueStyleTexts[1]);
+  const whitePromise = whiteSheet.replace(whiteStyleText);
+  const yellowPromise = yellowSheet.replace(yellowStyleText);
+  return [greenPromise, redPromise, bluePromise, whitePromise, yellowPromise];
+}
+
+promise_test(() => {
+  return Promise.all(createAllSheetsPromise()).then(values => {
+    const greenStyleSheet = values[0];
+    const redStyleSheet = values[1];
+    const blueStyleSheet = values[2];
+    const whiteStyleSheet = values[3];
+    const yellowStyleSheet = values[4];
+
+    // Lists of style sheets can be created, assigned and read.
+
+    // disabled stylesheets aren't applied
+    document.adoptedStyleSheets = [whiteStyleSheet];
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    // disable dsheets don't block other styles from applying
+    document.adoptedStyleSheets = [greenStyleSheet, blueStyleSheet];
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 128, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    document.adoptedStyleSheets = [redStyleSheet, yellowStyleSheet];
+
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(255, 255, 0)");
+
+    document.adoptedStyleSheets = [redStyleSheet, yellowStyleSheet, greenStyleSheet, blueStyleSheet];
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 128, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(255, 255, 0)");
+    document.adoptedStyleSheets = [];
+  });
+}, 'Constructed style sheets can be applied on document');
+
+promise_test(() => {
+  return Promise.all(createAllSheetsPromise()).then(values => {
+    const greenStyleSheet = values[0];
+    const redStyleSheet = values[1];
+    const blueStyleSheet = values[2];
+    const whiteStyleSheet = values[3];
+    const yellowStyleSheet = values[4];
+    shadowRoot.adoptedStyleSheets = [whiteStyleSheet];
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowShadowSpan).color, "rgb(0, 0, 0)");
+
+    shadowRoot.adoptedStyleSheets = [greenStyleSheet, blueStyleSheet];
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 128, 0)");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowShadowSpan).color, "rgb(0, 0, 0)");
+
+    shadowRoot.adoptedStyleSheets = [redStyleSheet, yellowStyleSheet];
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteShadowSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowShadowSpan).color, "rgb(255, 255, 0)");
+
+    shadowRoot.adoptedStyleSheets = [redStyleSheet, yellowStyleSheet, greenStyleSheet, blueStyleSheet];
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 128, 0)");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteShadowSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowShadowSpan).color, "rgb(255, 255, 0)");
+  });
+}, 'Constructed style sheets can be applied on shadow root');
+
+promise_test(() => {
+  return Promise.all(createAllSheetsPromise()).then(values => {
+    const greenStyleSheet = values[0];
+    const redStyleSheet = values[1];
+    shadowRoot.adoptedStyleSheets = [greenStyleSheet];
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 128, 0)", "Style applies connected");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(0, 0, 0)", "Style applies when connected");
+    let hostParent = shadowHost.parentNode;
+    hostParent.removeChild(shadowHost);
+    assert_equals(getComputedStyle(greenShadowSpan).color, "", "Style doesn't apply when detached");
+    assert_equals(getComputedStyle(redShadowSpan).color, "", "Style doesn't apply when detached");
+    shadowRoot.adoptedStyleSheets = [redStyleSheet, greenStyleSheet];
+    hostParent.appendChild(shadowHost);
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 128, 0)", "Style applies after reattach");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(255, 0, 0)", "Style applies after reattach");
+  });
+}, 'Re-attaching shadow host with adopted stylesheets work');
+
+test(() => {
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync(":host { color: red; }");
+  const host = document.createElement("div");
+  let sr = host.attachShadow({mode: "open"});
+  sr.adoptedStyleSheets = [sheet];
+  document.body.appendChild(host);
+  assert_equals(getComputedStyle(host).color, "rgb(255, 0, 0)", "Style applies when connected");
+  sheet.replaceSync(":host { color: blue; }");
+  assert_equals(getComputedStyle(host).color, "rgb(0, 0, 255)", "Style update applies when connected");
+}, 'Attaching a shadow root that already has adopted stylesheets work');
+
+test(() => {
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync(":host([red]) { color: red; } :host(.blue) { color: blue; }");
+  const host = document.createElement("div");
+  host.toggleAttribute("red");
+  document.body.appendChild(host);
+  assert_equals(getComputedStyle(host).color, "rgb(0, 0, 0)", "No style applies yet");
+
+  let sr = host.attachShadow({mode: "open"});
+  sr.adoptedStyleSheets = [sheet];
+
+  assert_equals(getComputedStyle(host).color, "rgb(255, 0, 0)", "Style applies after adding style");
+  document.body.removeChild(host);
+  document.body.appendChild(host);
+  assert_equals(getComputedStyle(host).color, "rgb(255, 0, 0)", "Style applies after reattachment");
+  host.toggleAttribute("red");
+  assert_equals(getComputedStyle(host).color, "rgb(0, 0, 0)", "Attribute updates to the element after reattachment apply");
+  host.classList.toggle("blue");
+  assert_equals(getComputedStyle(host).color, "rgb(0, 0, 255)", "Class updates to the element after reattachment apply");
+
+}, "Re-attaching shadow host and updating attributes work");
+
+promise_test(() => {
+  const plainSheet = new CSSStyleSheet();
+  const redStyleSheetPromise = plainSheet.replace(redStyleTexts[0]);
+  return redStyleSheetPromise.then(function(redStyleSheet) {
+    document.adoptedStyleSheets = [redStyleSheet];
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    redStyleSheet.insertRule(redStyleTexts[1]);
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    redStyleSheet.deleteRule(1);
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    redStyleSheet.cssRules[0].style.color = "white";
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(255, 255, 255)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+});
+}, 'Changes to constructed stylesheets through CSSOM is reflected');
+
+promise_test(() => {
+  const plainSheet = new CSSStyleSheet();
+  const redStyleSheetPromise = plainSheet.replace(redStyleTexts[0]);
+  return redStyleSheetPromise.then(function(redStyleSheet) {
+    document.adoptedStyleSheets = [redStyleSheet];
+    shadowRoot.adoptedStyleSheets = [redStyleSheet];
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowShadowSpan).color, "rgb(0, 0, 0)");
+
+    shadowRoot.adoptedStyleSheets[0].insertRule(redStyleTexts[1]);
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    assert_equals(getComputedStyle(greenShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redShadowSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueShadowSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteShadowSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowShadowSpan).color, "rgb(0, 0, 0)");
+    document.adoptedStyleSheets = [];
+  });
+}, 'Constructed stylesheet can be used and modified in multiple TreeScopes');
+
+promise_test(() => {
+  const iframe = document.createElement("iframe");
+  document.body.appendChild(iframe);
+  const thirdDiv = firstDiv.cloneNode(true);
+  iframe.contentDocument.body.appendChild(thirdDiv);
+  const greenIframeSpan = thirdDiv.children[0];
+  const redIframeSpan = thirdDiv.children[1];
+  const blueIframeSpan = thirdDiv.children[2];
+  const whiteIframeSpan = thirdDiv.children[3];
+  const yellowIframeSpan = thirdDiv.children[4];
+
+  const plainSheet = new CSSStyleSheet();
+  const redStyleSheetPromise = plainSheet.replace(redStyleTexts[0]);
+  return redStyleSheetPromise.then(function(redStyleSheet) {
+    assert_throws_dom(
+      'NotAllowedError',
+      iframe.contentWindow.DOMException,
+      () => { iframe.contentDocument.adoptedStyleSheets = [redStyleSheet]; }
+    );
+    assert_equals(getComputedStyle(greenIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowIframeSpan).color, "rgb(0, 0, 0)");
+
+    document.adoptedStyleSheets = [redStyleSheet];
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    document.adoptedStyleSheets[0].insertRule(redStyleTexts[1]);
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+  });
+}, 'Stylesheets constructed on the main Document cannot be used in iframes');
+
+promise_test(async () => {
+  const iframe = document.createElement("iframe");
+  const iframeLoaded = new Promise(resolve => iframe.addEventListener("load", resolve));
+  document.body.appendChild(iframe);
+  await iframeLoaded;
+  const thirdDiv = firstDiv.cloneNode(true);
+  iframe.contentDocument.body.appendChild(thirdDiv);
+  const greenIframeSpan = thirdDiv.children[0];
+  const redIframeSpan = thirdDiv.children[1];
+  const blueIframeSpan = thirdDiv.children[2];
+  const whiteIframeSpan = thirdDiv.children[3];
+  const yellowIframeSpan = thirdDiv.children[4];
+
+  // Make sure both the main Document and the iframe are not styled
+  const emptyStyleSheet = new CSSStyleSheet();
+  document.adoptedStyleSheets = [emptyStyleSheet];
+  assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(redSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+  assert_equals(getComputedStyle(greenIframeSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(redIframeSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(blueIframeSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(whiteIframeSpan).color, "rgb(0, 0, 0)");
+  assert_equals(getComputedStyle(yellowIframeSpan).color, "rgb(0, 0, 0)");
+
+  const iframePlainSheet = new iframe.contentWindow.CSSStyleSheet();
+  const iframeRedStyleSheetPromise = iframePlainSheet.replace(redStyleTexts[0]);
+  return iframeRedStyleSheetPromise.then(function(iframeRedStyleSheet) {
+    assert_throws_dom('NotAllowedError', () => { document.adoptedStyleSheets = [iframeRedStyleSheet]; });
+    assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");
+
+    iframe.contentDocument.adoptedStyleSheets = [iframeRedStyleSheet];
+    assert_equals(getComputedStyle(greenIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redIframeSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(yellowIframeSpan).color, "rgb(0, 0, 0)");
+
+    iframe.contentDocument.adoptedStyleSheets[0].insertRule(redStyleTexts[1]);
+    assert_equals(getComputedStyle(greenIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(redIframeSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(blueIframeSpan).color, "rgb(0, 0, 0)");
+    assert_equals(getComputedStyle(whiteIframeSpan).color, "rgb(255, 0, 0)");
+    assert_equals(getComputedStyle(yellowIframeSpan).color, "rgb(0, 0, 0)");
+  });
+}, 'Stylesheet constructed on iframe cannot be used in the main Document');
+</script>
+
+<div id="divNonConstructed" class="nonConstructed">
+</div>
+
+<script>
+`use strict`;
+const shadowRootNonConstructed = divNonConstructed.attachShadow({mode:'open'})
+nonConstructedStyle = document.createElement("style");
+shadowRootNonConstructed.appendChild(nonConstructedStyle);
+nonConstructedStyle.sheet.insertRule(".nonConstructed { color: red; }", 0);
+const nonConstructedStyleSheet = nonConstructedStyle.sheet;
+
+test(() => {
+  assert_equals(getComputedStyle(divNonConstructed).color, "rgb(0, 0, 0)");
+  assert_throws_dom('NotAllowedError', () => { document.adoptedStyleSheets = [nonConstructedStyleSheet]; });
+}, 'Adding non-constructed stylesheet to AdoptedStyleSheets is not allowed when the owner document of the stylesheet is in the same document tree as the AdoptedStyleSheets');
+
+test(() => {
+  const iframe = document.createElement("iframe");
+  document.body.appendChild(iframe);
+  iframeDiv = iframe.contentDocument.createElement("div");
+  iframeDiv.classList.add("nonConstructed");
+  iframe.contentDocument.body.appendChild(iframeDiv);
+
+  assert_equals(getComputedStyle(iframeDiv).color, "rgb(0, 0, 0)");
+  assert_throws_dom('NotAllowedError', iframe.contentWindow.DOMException, () => {
+    iframe.contentDocument.adoptedStyleSheets = [nonConstructedStyleSheet];
+  });
+  assert_equals(getComputedStyle(iframeDiv).color, "rgb(0, 0, 0)");
+
+  iframeStyle = iframe.contentDocument.createElement("style");
+  iframe.contentDocument.body.appendChild(iframeStyle);
+  iframeStyle.sheet.insertRule(".nonConstructedSpan { color: red; }");
+  const iframeStyleSheet = iframeStyle.sheet;
+  nonConstructedSpan = document.createElement("span");
+  nonConstructedSpan.classList.add(".nonConstructedSpan");
+  divNonConstructed.appendChild(nonConstructedSpan);
+
+  assert_equals(getComputedStyle(iframeDiv).color, "rgb(0, 0, 0)");
+  assert_throws_dom('NotAllowedError', () => { document.adoptedStyleSheets = [iframeStyleSheet]; });
+  assert_equals(getComputedStyle(iframeDiv).color, "rgb(0, 0, 0)");
+}, 'Adding non-constructed stylesheet to AdoptedStyleSheets is not allowed when the owner document of the stylesheet and the AdoptedStyleSheets are in different document trees');
+
+function attachShadowDiv(host) {
+  const shadowRoot = host.attachShadow({mode: 'open'});
+  const shadowDiv = document.createElement("div");
+  shadowRoot.appendChild(shadowDiv);
+  return shadowDiv;
+}
+
+test(() => {
+  const sheet = new CSSStyleSheet();
+  assert_equals(sheet.cssRules.length, 0);
+
+  sheet.replaceSync(redStyleTexts[0])
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(redStyleTexts[0], sheet.cssRules[0].cssText);
+
+  sheet.replaceSync(redStyleTexts[1]);
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(redStyleTexts[1], sheet.cssRules[0].cssText);
+}, 'CSSStyleSheet.replaceSync replaces stylesheet text synchronously');
+
+test(() => {
+  // Attach a div inside a shadow root with the class ".red".
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  shadowDiv.classList.add("red");
+  // Create empty stylesheet.
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  // Replace the stylesheet text that will color it red.
+  sheet.replaceSync(redStyleTexts[0]);
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)");
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(sheet.cssRules[0].cssText, redStyleTexts[0]);
+  sheet.insertRule(redStyleTexts[1]);
+  assert_equals(sheet.cssRules.length, 2);
+  assert_equals(sheet.cssRules[0].cssText, redStyleTexts[1]);
+}, 'CSSStyleSheet.replaceSync correctly updates the style of its adopters synchronously');
+
+test(() => {
+  // Attach a div inside a shadow root with the class ".red".
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  shadowDiv.classList.add("target");
+
+  // Create empty stylesheet.
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+
+  // Replace the stylesheet text that will color it red.
+  sheet.replaceSync(".target { color: red; }");
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)");
+
+  // Create a style element that will set colors to white.
+  const style = document.createElement("style");
+  style.textContent = ".target { color: white; }";
+  span.shadowRoot.appendChild(style)
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)", "non-adopted styles should be ordered before adopted styles");
+
+  span.shadowRoot.adoptedStyleSheets = [];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 255, 255)", "with no adopted styles in conflict, the non-adopted style should take effect");
+
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)", "the adopted style should be ordered after the non-adopted style");
+
+  sheet.disabled = true;
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 255, 255)", "with the adopted sheet disabled, the non-adopted style should take effect");
+
+  sheet.disabled = false;
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)", "the adopted sheet re-enabled, it should take effect again");
+}, 'Adopted sheets are ordered after non-adopted sheets in the shadow root')
+
+test(() => {
+  // Attach a div inside a shadow root with the class ".red".
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  span.classList.add("target");
+
+  // Create empty stylesheet.
+  const sheet = new CSSStyleSheet();
+  document.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(span).color, "rgb(0, 0, 0)");
+
+  // Replace the stylesheet text that will color it red.
+  sheet.replaceSync(".target { color: red; }");
+  assert_equals(getComputedStyle(span).color, "rgb(255, 0, 0)");
+
+  // Create a style element that will set colors to white.
+  const style = document.createElement("style");
+  style.textContent = ".target { color: white; }";
+  span.appendChild(style)
+  assert_equals(getComputedStyle(span).color, "rgb(255, 0, 0)", "non-adopted styles should be ordered before adopted styles");
+
+  document.adoptedStyleSheets = [];
+  assert_equals(getComputedStyle(span).color, "rgb(255, 255, 255)", "with no adopted styles in conflict, the non-adopted style should take effect");
+
+  document.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(span).color, "rgb(255, 0, 0)", "the adopted style should be ordered after the non-adopted style");
+
+  sheet.disabled = true;
+  assert_equals(getComputedStyle(span).color, "rgb(255, 255, 255)", "with the adopted sheet disabled, the non-adopted style should take effect");
+
+  sheet.disabled = false;
+  assert_equals(getComputedStyle(span).color, "rgb(255, 0, 0)", "the adopted sheet re-enabled, it should take effect again")
+}, 'Adopted sheets are ordered after non-adopted sheets in the document')
+
+const import_text = '@import url("support/constructable-import.css");';
+
+test(() => {
+  assert_throws_dom("SyntaxError", () => { (new CSSStyleSheet).insertRule(import_text) });
+}, 'Inserting an @import rule through insertRule on a constructed stylesheet throws an exception');
+
+promise_test(t => {
+    const importUrl = "support/constructable-import.css";
+    const sheet = new CSSStyleSheet();
+
+    sheet.replaceSync(`@import url("${importUrl}");`);
+
+    const timeAfterReplaceSync = performance.now();
+    let link = document.createElement("link");
+    link.rel = "stylesheet";
+    link.href = importUrl;
+
+    return new Promise(resolve => {
+      link.addEventListener("error", t.unreached_func("Load shouldn't fail"));
+      link.addEventListener("load", t.step_func(() => {
+        let entries = window.performance.getEntriesByType('resource').filter(entry => entry.name.includes(importUrl));
+        assert_equals(entries.length, 1, "There should be only one entry for the import URL");
+        assert_greater_than_equal(entries[0].startTime, timeAfterReplaceSync, "The entry's start time should be after replaceSync threw");
+        link.remove();
+        resolve();
+      }));
+      document.body.appendChild(link);
+    });
+}, "CSSStyleSheet.replaceSync should not trigger any loads from @import rules")
+
+promise_test(() => {
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  // Replace and assert that the imported rule is NOT applied.
+  const sheet_promise = sheet.replace(import_text);
+  return sheet_promise.then((sheet) => {
+    // replace() ignores @import rules:
+    assert_equals(sheet.cssRules.length, 0);
+    assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  }).catch((reason) => {
+    assert_unreached(`Promise was rejected (${reason}) when it should have been resolved`);
+  });
+}, 'CSSStyleSheet.replace allows, but ignores, import rule inside');
+
+promise_test(() => {
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  const targetSpan = document.createElement("span");
+  targetSpan.classList.add("target");
+  shadowDiv.appendChild(targetSpan);
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  // Replace and assert that the imported rule is NOT applied, but regular rule does apply.
+  const sheet_promise = sheet.replace(import_text + ".target { color: blue; }");
+  return sheet_promise.then((sheet) => {
+    assert_equals(sheet.cssRules.length, 1);
+    // @import not applied:
+    assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+    // regular rule applied:
+    assert_equals(getComputedStyle(targetSpan).color, "rgb(0, 0, 255)");
+  }).catch((reason) => {
+    assert_unreached(`Promise was rejected (${reason}) when it should have been resolved`);
+  });
+}, 'CSSStyleSheet.replace ignores @import rule but still loads other rules');
+
+test(() => {
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  // Replace and assert that the imported rule is NOT applied.
+  try {
+    sheet.replaceSync(import_text);
+    // replaceSync() ignores @import rules:
+    assert_equals(sheet.cssRules.length, 0);
+    assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+  } catch(reason) {
+    assert_unreached(`replaceSync threw an exception (${reason}) when it shouldn't have`);
+  }
+}, 'CSSStyleSheet.replaceSync allows, but ignores, import rule inside');
+
+promise_test(() => {
+  const sheet = new CSSStyleSheet();
+  const sheet_promise = sheet.replace("@import url('not-there.css');");
+
+  return sheet_promise.then((sheet) => {
+    // No exception here
+  }).catch((reason) => {
+    assert_unreached("Promise was rejected");
+  });
+}, 'CSSStyleSheet.replace does not reject on failed imports');
+
+test(() => {
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+
+  const newSpan = span.cloneNode(true);
+  assert_equals(newSpan.shadowRoot, null);
+}, 'Cloning a shadow host will not clone shadow root, and also adoptedStyleSheets');
+
+test(() => {
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+
+  const iframe = document.createElement("iframe");
+  document.body.appendChild(iframe);
+  const newSpan = iframe.contentDocument.importNode(span, true);
+  iframe.contentDocument.body.appendChild(newSpan);
+  assert_equals(newSpan.shadowRoot, null);
+}, 'Importing a shadow host will not copy shadow root, and also adoptedStyleSheets');
+
+test(() => {
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("* { color: red; }");
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)");
+
+  document.adoptNode(span);
+  assert_equals(span.shadowRoot.adoptedStyleSheets.length, 1);
+  assert_equals(span.shadowRoot.adoptedStyleSheets[0], sheet);
+
+  const iframe = document.createElement("iframe");
+  document.body.appendChild(iframe);
+  iframe.contentDocument.adoptNode(span);
+  iframe.contentDocument.body.appendChild(span);
+  assert_not_equals(span.shadowRoot, null);
+  assert_equals(span.shadowRoot.adoptedStyleSheets.length, 0);
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+}, 'Adopting a shadow host will empty adoptedStyleSheets if adopting to a different document');
+
+test(() => {
+  const span = document.createElement("span");
+  const div = document.createElement("div");
+  thirdSection.appendChild(span);
+  span.appendChild(div);
+  const shadowDiv = attachShadowDiv(div);
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("* { color: red; }");
+  div.shadowRoot.adoptedStyleSheets = [sheet];
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)");
+
+  document.adoptNode(span);
+  assert_equals(div.shadowRoot.adoptedStyleSheets.length, 1);
+  assert_equals(div.shadowRoot.adoptedStyleSheets[0], sheet);
+
+  const iframe = document.createElement("iframe");
+  document.body.appendChild(iframe);
+  iframe.contentDocument.adoptNode(span);
+  iframe.contentDocument.body.appendChild(span);
+  assert_not_equals(div.shadowRoot, null);
+  assert_equals(div.shadowRoot.adoptedStyleSheets.length, 0);
+  assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
+}, `Adopting a shadow host's ancestor will empty adoptedStyleSheets if adopting to a different document`);
+
+test(() => {
+  const host = document.createElement("div");
+  const root = host.attachShadow({mode: "open"});
+  root.adoptedStyleSheets = [new CSSStyleSheet()];
+  document.body.offsetTop;
+}, 'Forcing a style update after adding an adopted stylesheet on a disconnected shadow root should not crash.');
+
+test(() => {
+  const host = document.createElement("div");
+  thirdSection.appendChild(host);
+  const root = host.attachShadow({mode: "open"});
+  const sheet = new CSSStyleSheet();
+  root.adoptedStyleSheets = [sheet];
+  host.remove();
+  sheet.replaceSync('');
+}, 'Modifying an adopted stylesheet on a disconnected shadow root should not crash.');
+
+function currentLocation() {
+  const sections = location.href.split("/")
+  sections.pop();
+  return sections.join("/");
+}
+
+test(() => {
+  const span = document.createElement("span");
+  thirdSection.appendChild(span);
+  const shadowDiv = attachShadowDiv(span);
+
+  const fileName = "example.png"
+  const fullPath = `${currentLocation()}/${fileName}`
+
+  const sheet = new CSSStyleSheet();
+  span.shadowRoot.adoptedStyleSheets = [sheet];
+
+  sheet.replaceSync(`* { background-image: url("${fileName}"); }`);
+  const styleFromRelative = getComputedStyle(shadowDiv).backgroundImage;
+
+  sheet.replaceSync(`* { background-image: url("${fullPath}"); }`);
+  const styleFromFull = getComputedStyle(shadowDiv).backgroundImage;
+
+  assert_equals(styleFromRelative, styleFromFull);
+}, "Constructing a sheet with the default base URL uses the constructor document's base URL for CSS rules");
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-modify-after-removal.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-modify-after-removal.html
new file mode 100644
index 0000000000..010c0b9328
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-modify-after-removal.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<title>CSS Test: CSSStyleSheet modifications after removal</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstylesheet-interface">
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-cssrule-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<iframe id="frm"></iframe>
+<iframe id="frm2"></iframe>
+<script>
+  test(() => {
+    frm.contentDocument.body.innerHTML = "<style>div {color:red}</style>";
+    let sheet = frm.contentDocument.querySelector("style").sheet;
+    assert_equals(sheet.cssRules.length, 1);
+    frm.remove();
+    document.body.offsetTop;
+    sheet.insertRule("span {color: green}", 0);
+    assert_equals(sheet.cssRules.length, 2);
+  }, "Modify sheet from removed iframe");
+
+  test(() => {
+    frm2.contentWindow.eval("let sheet = new CSSStyleSheet(); document.adoptedStyleSheets = [ sheet ];");
+    let sheet = frm2.contentDocument.adoptedStyleSheets[0];
+    assert_equals(sheet.cssRules.length, 0);
+    frm2.remove();
+    document.body.offsetTop;
+    sheet.insertRule("span {color: green}", 0);
+    assert_equals(sheet.cssRules.length, 1);
+  }, "Modify constructed sheet from removed iframe");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet-template-adoption.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet-template-adoption.html
new file mode 100644
index 0000000000..e014627ed8
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet-template-adoption.html
@@ -0,0 +1,58 @@
+<!doctype html>
+<title>adoptedStyleSheets should stay when adopting to/from template document</title>
+<link rel="author" title="Rakina Zata Amni" href="mailto:rakina@chromium.org">
+<link rel="help" href="https://wicg.github.io/construct-stylesheets/">
+<script src = '/resources/testharness.js'></script>
+<script src = '/resources/testharnessreport.js'></script>
+<body>
+<div id="host"></div>
+<template id='template'>
+  <template id='nested_template'></template>
+</template>
+</body>
+<script>
+test(function() {
+  let sheet = new CSSStyleSheet();
+  sheet.replaceSync("div { color: blue }");
+
+  let host = document.getElementById("host");
+  let root = host.attachShadow({ mode: "open" });
+  root.innerHTML = `<div></div>`;
+  root.adoptedStyleSheets = [sheet];
+
+  function assertAdoptedStyleSheet() {
+    assert_equals(host.ownerDocument, root.firstChild.ownerDocument, "Shadow root was not adopted?");
+    assert_equals(root.adoptedStyleSheets.length, 1);
+    assert_equals(root.adoptedStyleSheets[0], sheet);
+    if (root.ownerDocument == document) {
+      assert_equals(getComputedStyle(root.firstChild).color, "rgb(0, 0, 255)", "Sheet should apply");
+    }
+  }
+
+  assertAdoptedStyleSheet();
+
+  // adoptedStyleSheets is not cleared when adopted into a <template>.
+  const template = document.getElementById("template");
+  template.content.appendChild(host);
+
+  assert_not_equals(host.ownerDocument, document, "Should've been adopted");
+  assertAdoptedStyleSheet();
+
+  // adoptedStyleSheets is not cleared when adopted back in the main document.
+  document.body.appendChild(host);
+  assert_equals(host.ownerDocument, document, "Should've been re-adopted");
+  assertAdoptedStyleSheet();
+
+  // adoptedStyleSheets is not cleared when adopted into a nested <template>.
+  const nested_template = template.content.firstElementChild;
+  nested_template.content.appendChild(host);
+  assert_not_equals(host.ownerDocument, document, "Should've been adopted");
+  assertAdoptedStyleSheet();
+
+  // adoptedStyleSheets is cleared when adopted into an <iframe>.
+  const iframe = document.createElement("iframe");
+  document.body.appendChild(iframe);
+  iframe.contentDocument.body.appendChild(host);
+  assert_equals(root.adoptedStyleSheets.length, 0);
+}, "adoptedStyleSheets won'te be cleared when adopting into/from <template>s");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/CSSStyleSheet.html b/testing/web-platform/tests/css/cssom/CSSStyleSheet.html
new file mode 100644
index 0000000000..c744382b55
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/CSSStyleSheet.html
@@ -0,0 +1,132 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - CSSStyleSheet interface</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstylesheet-interface">
+    <link rel="help" href="https://drafts.csswg.org/cssom/#legacy-css-style-sheet-members">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style id="my-stylesheet">
+        body { width: 50%; }
+        #foo { height: 100px; }
+    </style>
+    <style id="empty-stylesheet"></style>
+
+    <script>
+    var styleSheet, emptyStyleSheet;
+
+    setup(function() {
+        styleSheet = document.styleSheets[0];
+        styleSheet.cssRules[0].randomProperty = 1;
+        styleSheet.cssRules[1].randomProperty = 2;
+        emptyStyleSheet = document.styleSheets[1];
+    });
+
+    test(function() {
+        assert_equals(styleSheet, document.getElementById("my-stylesheet").sheet, "CSSStyleSheet and LinkStyle's sheet attribute");
+        assert_equals(styleSheet.cssRules.length, 2, "CSSStyleSheet cssRules attribute");
+        assert_equals(styleSheet.cssRules[0].cssText, "body { width: 50%; }", "CSSStyleSheet cssRules attribute");
+        assert_equals(styleSheet.cssRules[1].cssText, "#foo { height: 100px; }", "CSSStyleSheet cssRules attribute");
+        assert_equals(styleSheet.cssRules[2], undefined, "CSSStyleSheet cssRules attribute");
+
+        assert_equals(emptyStyleSheet, document.getElementById("empty-stylesheet").sheet, "CSSStyleSheet and LinkStyle's sheet attribute");
+        assert_equals(emptyStyleSheet.cssRules.length, 0, "CSSStyleSheet cssRules attribute");
+    }, "preconditions");
+
+    test(function() {
+        styleSheet.insertRule("#bar { margin: 10px; }", 1);
+        assert_equals(styleSheet.cssRules.length, 3, "CSSStyleSheet cssRules attribute after insertRule function");
+        assert_equals(styleSheet.cssRules[0].cssText, "body { width: 50%; }", "CSSStyleSheet cssRules attribute");
+        assert_equals(styleSheet.cssRules[1].cssText, "#bar { margin: 10px; }", "CSSStyleSheet cssRules attribute after insertRule function");
+        assert_equals(styleSheet.cssRules[2].cssText, "#foo { height: 100px; }", "CSSStyleSheet cssRules attribute after insertRule function");
+        assert_equals(styleSheet.cssRules[0].randomProperty, 1, "[SameObject] cssRules attribute after insertRule function");
+        assert_equals(styleSheet.cssRules[2].randomProperty, 2, "[SameObject] cssRules attribute after insertRule function");
+    }, 'insertRule with #bar selector');
+
+    test(function() {
+        assert_throws_js(TypeError, function() { styleSheet.insertRule() });
+    }, 'insertRule with no argument throws');
+
+    test(function() {
+        assert_throws_dom("IndexSizeError", function() {
+            styleSheet.insertRule("#bar { margin: 10px; }", styleSheet.cssRules.length + 1)
+        });
+    }, 'insertRule with index greater than length throws');
+
+    test(function() {
+        styleSheet.deleteRule(1);
+        assert_equals(styleSheet.cssRules.length, 2, "CSSStyleSheet cssRules attribute after deleteRule function");
+        assert_equals(styleSheet.cssRules[0].cssText, "body { width: 50%; }", "CSSStyleSheet cssRules attribute after deleteRule function");
+        assert_equals(styleSheet.cssRules[1].cssText, "#foo { height: 100px; }", "CSSStyleSheet cssRules attribute after deleteRule function");
+        assert_equals(styleSheet.cssRules[2], undefined, "CSSStyleSheet cssRules attribute after deleteRule function");
+        assert_equals(styleSheet.cssRules[0].randomProperty, 1, "[SameObject] cssRules attribute after deleteRule function");
+        assert_equals(styleSheet.cssRules[1].randomProperty, 2, "[SameObject] cssRules attribute after deleteRule function");
+    }, 'deleteRule(1)');
+
+    test(function() {
+        assert_throws_js(TypeError, function() { styleSheet.deleteRule() });
+    }, 'deleteRule with no argument throws');
+
+    test(function() {
+        assert_throws_dom("IndexSizeError", function() { emptyStyleSheet.deleteRule(0) });
+    }, 'deleteRule on empty style sheet throws');
+
+    test(function() {
+        styleSheet.removeRule();
+        assert_equals(styleSheet.cssRules.length, 1, "CSSStyleSheet cssRules attribute after removeRule function");
+        assert_equals(styleSheet.cssRules[0].cssText, "#foo { height: 100px; }", "CSSStyleSheet cssRules attribute after removeRule function");
+    }, 'removeRule with no argument removes first rule');
+
+    test(function() {
+        assert_throws_dom("IndexSizeError", function() { emptyStyleSheet.removeRule(0) });
+    }, 'removeRule on empty style sheet throws');
+
+    test(function() {
+        assert_equals(styleSheet.addRule("@media all", "#foo { color: red }"), -1);
+        assert_equals(styleSheet.cssRules.length, 2, "CSSStyleSheet cssRules attribute after addRule function");
+        assert_true(styleSheet.cssRules[1] instanceof CSSMediaRule, "CSSStyleSheet addRule does some silly string concatenation");
+    }, 'addRule with @media rule');
+
+    test(function() {
+        styleSheet.removeRule(1);
+        assert_equals(styleSheet.cssRules.length, 1, "CSSStyleSheet cssRules attribute after removeRule function with index");
+        assert_equals(styleSheet.cssRules[0].cssText, "#foo { height: 100px; }", "CSSStyleSheet cssRules attribute after deleteRule function with index");
+    }, 'removeRule(1)');
+
+    test(function() {
+        assert_equals(styleSheet.addRule("#foo", "color: red"), -1);
+        assert_equals(styleSheet.cssRules.length, 2, "CSSStyleSheet cssRules attribute after addRule function with simple selector");
+        assert_equals(styleSheet.cssRules[1].cssText, "#foo { color: red; }", "CSSStyleSheet cssRules attribute after addRule function without index appends to the end");
+
+        assert_equals(styleSheet.addRule("#foo", "color: blue", 0), -1);
+        assert_equals(styleSheet.cssRules.length, 3, "CSSStyleSheet cssRules attribute after addRule function with simple selector with index");
+        assert_equals(styleSheet.cssRules[0].cssText, "#foo { color: blue; }", "addRule function with index performs an insertion");
+    }, 'addRule with #foo selectors');
+
+    test(function() {
+        assert_equals(styleSheet.addRule(), -1);
+        assert_equals(styleSheet.cssRules.length, 4, "CSSStyleSheet cssRules attribute after addRule function without arguments");
+        assert_equals(styleSheet.cssRules[3].cssText, "undefined { }", "addRule arguments default to undefined");
+    }, 'addRule with no argument adds "undefined" selector');
+
+    test(function() {
+        assert_throws_dom("IndexSizeError", function() {
+            styleSheet.addRule("#foo", "color: red", styleSheet.cssRules.length + 1);
+        });
+    }, 'addRule with index greater than length throws');
+
+    test(function() {
+        assert_equals(styleSheet.cssRules, styleSheet.rules);
+    }, "cssRules and rules are the same object");
+
+    test(function() {
+        assert_equals(styleSheet.cssRules, styleSheet.cssRules);
+    }, "cssRules returns the same object twice");
+
+    test(function() {
+        assert_equals(styleSheet.rules, styleSheet.rules);
+    }, "rules returns the same object twice");
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-001.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-001.html
new file mode 100644
index 0000000000..f2c5d0a545
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-001.html
@@ -0,0 +1,44 @@
+<!doctype html>
+<title>&lt;link disabled&gt;, HTMLLinkElement.disabled and CSSStyleSheet.disabled interactions</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+test(function() {
+  assert_equals(document.styleSheets.length, 0);
+  assert_applies(false);
+}, "<link disabled> prevents the stylesheet from being in document.styleSheets (from parser)");
+
+async_test(function(t) {
+  assert_true(link.disabled);
+
+  link.onload = t.step_func_done(function() {
+    assert_equals(document.styleSheets.length, 1);
+    let sheet = document.styleSheets[0];
+    assert_equals(sheet.ownerNode, link);
+    assert_applies(true);
+
+    link.disabled = true;
+    assert_equals(sheet.ownerNode, null);
+    assert_false(sheet.disabled);
+    assert_applies(false);
+    assert_true(link.hasAttribute("disabled"));
+
+    assert_equals(document.styleSheets.length, 0);
+    assert_applies(false);
+  });
+
+  link.disabled = false;
+  assert_true(!link.hasAttribute("disabled"));
+  assert_false(link.disabled);
+}, "HTMLLinkElement.disabled reflects the <link disabled> attribute, and behaves consistently");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-002.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-002.html
new file mode 100644
index 0000000000..34bd8182fe
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-002.html
@@ -0,0 +1,37 @@
+<!doctype html>
+<title>&lt;link disabled&gt;, HTMLLinkElement.disabled and CSSStyleSheet.disabled interactions (alternate)</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="alternate stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+async_test(function(t) {
+  assert_true(link.disabled);
+
+  link.onload = t.step_func_done(function() {
+    assert_equals(document.styleSheets.length, 1);
+    let sheet = document.styleSheets[0];
+    assert_equals(sheet.ownerNode, link);
+    assert_applies(true);
+
+    link.disabled = true;
+    assert_equals(sheet.ownerNode, null);
+    assert_false(sheet.disabled);
+    assert_applies(false);
+    assert_true(link.hasAttribute("disabled"));
+    assert_equals(document.styleSheets.length, 0);
+  });
+
+  link.disabled = false;
+  assert_true(!link.hasAttribute("disabled"));
+  assert_false(link.disabled);
+}, "HTMLLinkElement.disabled reflects the <link disabled> attribute, and behaves consistently, when the sheet is an alternate");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-003.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-003.html
new file mode 100644
index 0000000000..3d391bbaff
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-003.html
@@ -0,0 +1,31 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state persists after getting disconnected from the tree</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="alternate stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+async_test(function(t) {
+  assert_true(link.disabled);
+  link.disabled = false;
+  assert_false(link.disabled);
+  assert_true(!link.hasAttribute("disabled"));
+  link.remove();
+
+  link.onload = t.step_func_done(function() {
+    assert_equals(document.styleSheets.length, 1);
+    let sheet = document.styleSheets[0];
+    assert_equals(sheet.ownerNode, link);
+    assert_applies(true);
+  });
+
+  document.head.appendChild(link);
+}, "HTMLLinkElement.disabled's explicitly enabled state persists when disconnected and connected again");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-004.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-004.html
new file mode 100644
index 0000000000..f163f1d68e
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-004.html
@@ -0,0 +1,39 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state doesn't persist for clones</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="alternate stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+async_test(function(t) {
+  link.remove();
+  link.disabled = false; // `link` is explicitly enabled.
+
+  let clonesLoaded = 0;
+
+  for (let shallow of [true, false]) {
+    const clone = link.cloneNode(shallow);
+    clone.onload = t.step_func(function() {
+      assert_false(link.disabled);
+      // Even though it's not disabled, it still doesn't apply, since it's an alternate.
+      assert_applies(false);
+      if (++clonesLoaded == 2) {
+        link.onload = t.step_func_done(function() {
+          assert_false(link.disabled);
+          assert_applies(true); // `link` is still explicitly enabled.
+        });
+        document.head.appendChild(link);
+      }
+    });
+    document.head.appendChild(clone);
+  }
+}, "HTMLLinkElement.disabled's explicitly enabled state doesn't persist on clones");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-005.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-005.html
new file mode 100644
index 0000000000..76de206327
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-005.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" persists across rel changes</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="yadayada" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+async_test(function(t) {
+  link.onload = t.step_func_done(function() {
+    assert_applies(true);
+    link.setAttribute("rel", "alternate stylesheet");
+    assert_applies(true);
+    assert_false(link.disabled);
+  });
+  link.disabled = false;
+  link.setAttribute("rel", "stylesheet");
+}, "HTMLLinkElement.disabled's explicitly enabled state persists regardless of rel");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-006.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-006.html
new file mode 100644
index 0000000000..02749a2e11
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-006.html
@@ -0,0 +1,25 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state isn't magically set from the setter</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+async_test(function(t) {
+  const link = document.createElement("link");
+  link.setAttribute("rel", "alternate stylesheet");
+  link.setAttribute("title", "alt");
+  link.href = "data:text/css,html { background: green }";
+  link.disabled = false; // This should do nothing, and is the point of this test.
+  link.onload = t.step_func_done(function() {
+    assert_applies(false); // Should not apply, since it's an alternate that hasn't been enabled.
+    assert_false(link.disabled);
+  });
+  document.head.appendChild(link);
+}, "HTMLLinkElement.disabled setter does nothing if the attribute isn't present already.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-007.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-007.html
new file mode 100644
index 0000000000..9a695a964c
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-007.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state works when set explicitly back and forth</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+async_test(function(t) {
+  const link = document.createElement("link");
+  link.setAttribute("rel", "alternate stylesheet");
+  link.setAttribute("title", "alt");
+  link.href = "data:text/css,html { background: green }";
+  link.disabled = true;
+  link.disabled = false; // This should make it "explicitly enabled".
+  link.onload = t.step_func_done(function() {
+    assert_applies(true); // Should apply, since it's explicitly enabled.
+    assert_false(link.disabled);
+  });
+  document.head.appendChild(link);
+}, "HTMLLinkElement.disabled setter sets the explicitly enabled state if toggled back and forth.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate-ref.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate-ref.html
new file mode 100644
index 0000000000..5d87bfdaf5
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate-ref.html
@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<title>CSS Test Reference</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<style>
+  html { background: green }
+</style>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate.html
new file mode 100644
index 0000000000..f1457c0dcf
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>CSS Test: alternate stylesheets can be disabled by HTMLLinkElement.disabled if they have the disabled attribute already</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#attr-link-disabled">
+<link rel="match" href="HTMLLinkElement-disabled-alternate-ref.html">
+<link title="alt" rel="alternate stylesheet" href="data:text/css,html { background: green }" disabled onload="document.documentElement.className = ''">
+<script>
+  onload = function() {
+    const link = document.querySelector("link[rel='alternate stylesheet']");
+    link.disabled = false;
+  }
+</script>
diff --git a/testing/web-platform/tests/css/cssom/META.yml b/testing/web-platform/tests/css/cssom/META.yml
new file mode 100644
index 0000000000..3250801f64
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/META.yml
@@ -0,0 +1,5 @@
+spec: https://drafts.csswg.org/cssom/
+suggested_reviewers:
+  - dbaron
+  - plinss
+  - lilles
diff --git a/testing/web-platform/tests/css/cssom/MediaList.html b/testing/web-platform/tests/css/cssom/MediaList.html
new file mode 100644
index 0000000000..8308ee89e4
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/MediaList.html
@@ -0,0 +1,55 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - MediaList interface</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-medialist-interface">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style>
+        @media screen and (min-width: 480px), print, projection {}
+    </style>
+
+    <script>
+    test(function () {
+        var media = document.styleSheets[0].cssRules[0].media;
+        assert_equals(media.length, 3, "MediaList length attribute");
+        assert_equals(media.mediaText, "screen and (min-width: 480px), print, projection", "MediaList mediaText attribute");
+        assert_equals(media.toString(), "screen and (min-width: 480px), print, projection", "MediaList toString method");
+        assert_equals(media[0], "screen and (min-width: 480px)", "MediaList indexed getter");
+        assert_equals(media[1], "print", "MediaList indexed getter");
+        assert_equals(media[2], "projection", "MediaList indexed getter");
+        assert_equals(media[3], undefined, "MediaList indexed getter with out of range");
+        assert_equals(media.item(0), "screen and (min-width: 480px)", "MediaList item method");
+        assert_equals(media.item(3), null, "MediaList item method");
+
+        media.deleteMedium("print");
+        assert_equals(media.length, 2, "MediaList length attribute after delete method");
+        assert_equals(media.mediaText, "screen and (min-width: 480px), projection", "MediaList mediaText attribute after delete method");
+        assert_equals(media.toString(), "screen and (min-width: 480px), projection", "MediaList toString method after delete method");
+        assert_equals(media[1], "projection", "MediaList indexed getter after delete method");
+        assert_equals(media[2], undefined, "MediaList indexed getter with out of range after delete method");
+        assert_equals(media.item(1), "projection", "MediaList indexed getter after delete method");
+        assert_equals(media.item(2), null, "MediaList item method after delete method");
+
+        media.appendMedium("speech");
+        assert_equals(media.length, 3, "MediaList length attribute after append method");
+        assert_equals(media.mediaText, "screen and (min-width: 480px), projection, speech", "MediaList mediaText attribute after append method");
+        assert_equals(media.toString(), "screen and (min-width: 480px), projection, speech", "MediaList toString method after append method");
+        assert_equals(media[1], "projection", "MediaList indexed getter after append method");
+        assert_equals(media[2], "speech", "MediaList indexed getter after append method");
+        assert_equals(media[3], undefined, "MediaList indexed getter with out of range after append method");
+        assert_equals(media.item(2), "speech", "MediaList item method after append method");
+        assert_equals(media.item(3), null, "MediaList item method after append method");
+
+        media.mediaText = null;
+        assert_equals(media.mediaText, "", "MediaList mediaText attribute should be empty string in case of null");
+        assert_equals(media.toString(), "", "MediaList toString method should be empty string in case of null");
+
+        var rule = document.styleSheets[0].cssRules[0];
+        rule.media = "speech";
+        assert_equals(rule.media.mediaText, "speech", "MediaList mediaText attribute should be updated");
+    });
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/MediaList2.xhtml b/testing/web-platform/tests/css/cssom/MediaList2.xhtml
new file mode 100644
index 0000000000..73acbdec25
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/MediaList2.xhtml
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+<title>CSS Test: the MediaList interface</title>
+<link rel="author" title="Ms2ger" href="mailto:Ms2ger@gmail.com"/>
+<link rel="help" href="http://www.w3.org/TR/cssom-1/#the-medialist-interface"/>
+<link rel="help" href="http://dev.w3.org/2006/webapi/WebIDL/#getownproperty"/>
+<style media="screen, print" id="test-style"></style>
+<script src="/resources/testharness.js"/>
+<script src="/resources/testharnessreport.js"/>
+</head>
+<body>
+<div id="log"/>
+<script>
+test(function() {
+  var ss = document.styleSheets[0];
+  assert_equals(ss.ownerNode.id, "test-style", "Got the wrong style element");
+
+  var media = ss.media;
+  test(function() {
+    // https://www.w3.org/Bugs/Public/show_bug.cgi?id=17526
+    assert_equals(media.mediaText, "screen, print", "Serialization should be \"screen, print\"");
+  }, "MediaList.mediaText");
+
+  test(function() {
+    assert_equals(media.length, 2, "Got wrong number of media");
+  }, "MediaList.length");
+
+  test(function() {
+    assert_equals(media[-1], undefined, "media[-1] should return undefined");
+    assert_equals(media[0], "screen", "media[0] should return \"screen\"");
+    assert_equals(media[1], "print", "media[1] should return \"print\"");
+    assert_equals(media[2], undefined, "media[2] should return undefined");
+  }, "MediaList getter");
+
+  test(function() {
+    assert_equals(media.item(-1), null, "media.item(-1) should return null");
+    assert_equals(media.item(0), "screen", "media.item(0) should return \"screen\"");
+    assert_equals(media.item(1), "print", "media.item(1) should return \"print\"");
+    assert_equals(media.item(2), null, "media.item(2) should return null");
+  }, "MediaList.item");
+}, "MediaList");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/MutationObserver-style.html b/testing/web-platform/tests/css/cssom/MutationObserver-style.html
new file mode 100644
index 0000000000..673822d11e
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/MutationObserver-style.html
@@ -0,0 +1,43 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Observer notifications when updating styles</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<body>
+<script>
+  "use strict";
+
+  promise_test(async () => {
+    let called = 0;
+    const el = document.createElement("div");
+    document.body.appendChild(el);
+    const m = new MutationObserver(() => {
+      called++;
+    });
+    m.observe(el, { attributes: true });
+    el.style.height = "100px";
+    await Promise.resolve();
+    assert_equals(called, 1, "times callback called");
+    el.style.height = "100px";
+    await Promise.resolve();
+    assert_equals(called, 1, "times callback called");
+  }, "Updating style property with the same value does not trigger an observation callback");
+
+  promise_test(async () => {
+    let called = 0;
+    const el = document.createElement("div");
+    document.body.appendChild(el);
+    const m = new MutationObserver(() => {
+      called++;
+    });
+    m.observe(el, { attributes: true });
+    el.style.cssText = "height:100px";
+    await Promise.resolve();
+    assert_equals(called, 1, "times callback called");
+    el.style.cssText = "height:100px";
+    await Promise.resolve();
+    assert_equals(called, 2, "times callback called");
+  }, "Updating cssText triggers an observation callback even if the value is the same");
+</script>
+
diff --git a/testing/web-platform/tests/css/cssom/StyleSheetList.html b/testing/web-platform/tests/css/cssom/StyleSheetList.html
new file mode 100644
index 0000000000..bb7d2ff4e3
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/StyleSheetList.html
@@ -0,0 +1,34 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - StyleSheetList interface</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-stylesheetlist-interface">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style>
+        body { width: 50%; }
+    </style>
+    <style>
+        #foo { width: 10%; }
+        #bar { height: 100px; }
+    </style>
+    <script>
+    test(function () {
+        var styleSheets = document.styleSheets;
+        assert_equals(styleSheets.length, 2, "StyleSheetList length attribute");
+        assert_equals(styleSheets[0].cssRules.length, 1, "StyleSheetList indexed getter length attribute");
+        assert_equals(styleSheets[1].cssRules.length, 2, "StyleSheetList indexed getter length attribute");
+        assert_equals(styleSheets[2], undefined, "StyleSheetList indexed getter length attribute");
+        assert_equals(styleSheets.item(0).cssRules.length, 1, "StyleSheetList item function length attribute");
+        assert_equals(styleSheets.item(1).cssRules.length, 2, "StyleSheetList item function length attribute");
+        assert_equals(styleSheets.item(2), null, "StyleSheetList item function length attribute");
+
+        styleSheets[0].randomProperty = 1;
+        var style = document.createElement("style");
+        document.head.appendChild(style);
+        assert_equals(styleSheets[0].randomProperty, 1, "[SameObject] StyleSheetList");
+    });
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/adoptedstylesheets-observablearray.html b/testing/web-platform/tests/css/cssom/adoptedstylesheets-observablearray.html
new file mode 100644
index 0000000000..783a054163
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/adoptedstylesheets-observablearray.html
@@ -0,0 +1,92 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Adoptedstylesheets as ObservableArray</title>
+<link rel="author" href="mailto:masonf@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<span id=target>Test Span</span>
+<div id="divNonConstructed" class="nonConstructed"></div>
+<style>
+  #target {background-color: red;}
+</style>
+
+<script>
+
+const shadowRootNonConstructed = divNonConstructed.attachShadow({mode:'open'})
+nonConstructedStyle = document.createElement("style");
+shadowRootNonConstructed.appendChild(nonConstructedStyle);
+nonConstructedStyle.sheet.insertRule(".nonConstructed { color: red; }", 0);
+const nonConstructedStyleSheet = nonConstructedStyle.sheet;
+
+function assert_is(targetStyle, color) {
+  assert_equals(targetStyle.getPropertyValue('background-color'), color);
+}
+
+function testRoot(d, targetStyle) {
+  const red = 'rgb(255, 0, 0)';
+  const green = 'rgb(0, 255, 0)';
+  const blue = 'rgb(0, 0, 255)';
+
+  const sheet1 = new CSSStyleSheet();
+  sheet1.replaceSync('#target {background-color:lime !important;}');
+  const sheet2 = new CSSStyleSheet();
+  sheet2.replaceSync('#target {background-color:blue !important;}');
+  assert_equals(d.adoptedStyleSheets.length, 0);
+  assert_is(targetStyle, red);
+
+  d.adoptedStyleSheets = [sheet1];
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, green);
+
+  d.adoptedStyleSheets.push(sheet2);
+  assert_equals(d.adoptedStyleSheets.length, 2);
+  assert_is(targetStyle, blue);
+
+  d.adoptedStyleSheets.pop();
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, green);
+
+  d.adoptedStyleSheets.push(sheet2);
+  d.adoptedStyleSheets.reverse();
+  assert_equals(d.adoptedStyleSheets.length, 2);
+  assert_is(targetStyle, green);
+
+  d.adoptedStyleSheets.splice(1, 1);
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, blue);
+  d.adoptedStyleSheets.splice(0, 1, sheet1);
+  assert_equals(d.adoptedStyleSheets.length, 1);
+  assert_is(targetStyle, green);
+
+  // Adding non-constructed stylesheet to AdoptedStyleSheets is not allowed.
+  assert_throws_dom('NotAllowedError', () => { document.adoptedStyleSheets.push(nonConstructedStyleSheet); });
+
+  assert_throws_js(TypeError, () => { document.adoptedStyleSheets.push("foo"); });
+}
+
+test(function() {
+  const target = document.querySelector('#target');
+  const targetStyle = window.getComputedStyle(target);
+  testRoot(document, targetStyle);
+}, "document.adoptedStyleSheets should allow mutation in-place");
+
+test(function() {
+  const host = document.createElement('div');
+  document.body.appendChild(host);
+  const shadow = host.attachShadow({mode: 'open'});
+  shadow.innerHTML = '<span id=target>Test Shadow Span</span><style>#target{background-color: red;}</style>';
+  const target = shadow.querySelector('#target');
+  const targetStyle = window.getComputedStyle(target);
+  testRoot(shadow, targetStyle);
+}, "shadowRoot.adoptedStyleSheets should allow mutation in-place");
+
+test(function() {
+  assert_true(Array.isArray(document.adoptedStyleSheets));
+  const host = document.createElement('div');
+  document.body.appendChild(host);
+  const shadow = host.attachShadow({mode: 'open'});
+  assert_true(Array.isArray(shadow.adoptedStyleSheets));
+}, "adoptedStyleSheets should return true for isArray()");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/at-namespace.html b/testing/web-platform/tests/css/cssom/at-namespace.html
new file mode 100644
index 0000000000..b1c76f3c58
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/at-namespace.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<title>CSS Test: @namespace in CSSOM is not severely broken</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1464865">
+<link rel="help" href="https://drafts.csswg.org/cssom/#insert-a-css-rule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style id="s">
+  div { color: green }
+</style>
+<div>Should be green</div>
+<script>
+test(function() {
+  assert_throws_dom("InvalidStateError", function() {
+    s.sheet.insertRule('@namespace myhtml url("http://www.w3.org/1999/xhtml")', 0);
+  });
+  assert_equals(s.sheet.cssRules.length, 1, "Shouldn't have been inserted");
+  assert_throws_dom("SyntaxError", function() {
+    s.sheet.insertRule("myhtml|div { color: red !important }", 0);
+  });
+  assert_equals(s.sheet.cssRules.length, 1);
+  assert_equals(
+    getComputedStyle(document.querySelector("div")).color,
+    "rgb(0, 128, 0)",
+    "Namespace shouldn't be registered"
+  );
+});
+</script>
+
diff --git a/testing/web-platform/tests/css/cssom/base-uri.html b/testing/web-platform/tests/css/cssom/base-uri.html
new file mode 100644
index 0000000000..b948d5a7f4
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/base-uri.html
@@ -0,0 +1,55 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM base URI is the document's base URI</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://html.spec.whatwg.org/#document-base-url">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1343919">
+<base href="/non-existent-base/">
+<script src='/resources/testharness.js'></script>
+<script src='/resources/testharnessreport.js'></script>
+<div id="target"></div>
+<script>
+const target = document.getElementById("target");
+const kRelativeURI = "url(something.png)";
+const kKeyframes = [
+  { backgroundImage: kRelativeURI },
+  { backgroundImage: kRelativeURI },
+];
+function assertBackground() {
+  const image = getComputedStyle(target).backgroundImage;
+  assert_true(image.includes("non-existent-base"), image);
+}
+function assertNoBackground() {
+  const image = getComputedStyle(target).backgroundImage;
+  assert_equals(image, "none");
+}
+
+promise_test(async function() {
+  target.style.backgroundImage = kRelativeURI;
+  assertBackground();
+  target.style.backgroundImage = "";
+  assertNoBackground();
+}, "setProperty");
+
+promise_test(async function() {
+  const keyframe = new KeyframeEffect(target, kKeyframes, 10000);
+  const animation = new Animation(keyframe, document.timeline);
+  animation.play();
+  await animation.ready;
+  assertBackground();
+  animation.cancel();
+  assertNoBackground();
+}, "KeyframeEffect constructor");
+
+promise_test(async function() {
+  const keyframe = new KeyframeEffect(target, [], 10000);
+  keyframe.setKeyframes(kKeyframes);
+  const animation = new Animation(keyframe, document.timeline);
+  animation.play();
+  await animation.ready;
+  assertBackground();
+  animation.cancel();
+  assertNoBackground();
+}, "KeyframeEffect.setKeyframes");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/border-shorthand-serialization.html b/testing/web-platform/tests/css/cssom/border-shorthand-serialization.html
new file mode 100644
index 0000000000..9b5821563c
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/border-shorthand-serialization.html
@@ -0,0 +1,42 @@
+<!doctype html>
+<html>
+<link rel="author" title="Erik Nordin" href="mailto:enordin@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/css-backgrounds-3/#propdef-border">
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#serialize-a-css-declaration-block">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1594241">
+<meta charset="utf-8">
+<title>serialization of border shorthand</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style id="target">
+    .a {
+        border-width: 1px;
+        border-style: solid;
+        border-color: black;
+    }
+    .b {
+        border-width: 1px;
+        border-style: solid;
+        border-color: black;
+        border-image: linear-gradient(white,black);
+    }
+    .c {
+        border: 1px solid black;
+    }
+</style>
+<script>
+    test(function() {
+        let rule = document.getElementById('target').sheet.cssRules[0];
+        assert_equals(rule.style.border, "", "border shorthand isn't serialized if border-image longhands are not initial");
+    }, "Declaration with border longhands is not serialized to a border shorthand declaration.");
+    test(function() {
+        let rule = document.getElementById('target').sheet.cssRules[1];
+        assert_equals(rule.style.border, "", "border shorthand isn't serialized if border-image longhands are not initial");
+    }, "Declaration with border longhands and border-image is not serialized to a border shorthand declaration.");
+    test(function() {
+        let rule = document.getElementById('target').sheet.cssRules[2];
+        assert_not_equals(rule.style.border, "", "border shorthand ");
+        assert_equals(rule.cssText, ".c { border: 1px solid black; }");
+    }, "Border shorthand is serialized correctly if all border-image-* are set to their initial specified values.");
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/caretPositionFromPoint-with-transformation.html b/testing/web-platform/tests/css/cssom/caretPositionFromPoint-with-transformation.html
new file mode 100644
index 0000000000..fcd02877ca
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/caretPositionFromPoint-with-transformation.html
@@ -0,0 +1,46 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>getCaretPositionFromPoint should return the correct offset even in iframes with transformation</title>
+<link rel="help" href="https://drafts.csswg.org/cssom-view-1/#dom-document-caretpositionfrompoint">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1546612">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  #actual {
+    transform: translateX(100px);
+  }
+</style>
+<iframe id="expected"></iframe>
+<br>
+<iframe id="actual"></iframe>
+<script>
+  const expectedFrame = document.getElementById("expected");
+  const actualFrame = document.getElementById("actual");
+
+  const getCaretPositionOffset = frame => {
+    const source = `<!doctype html><meta charset="utf-8"><h1>title</h1><p>paragraph</p>`
+
+    const elementCenter = elem => {
+      const rect = elem.getBoundingClientRect();
+      return [rect.x + rect.width / 2, rect.y + rect.height / 2];
+    };
+
+    return new Promise(resolve => {
+      frame.srcdoc = source;
+      frame.onload = () => {
+        const frameDoc = frame.contentDocument;
+        const {offset} = frameDoc.caretPositionFromPoint(
+          ...elementCenter(frameDoc.querySelector("h1"))
+        );
+        resolve(offset);
+      };
+    });
+  };
+
+  promise_test(async () => {
+    assert_equals(...await Promise.all([
+      getCaretPositionOffset(expectedFrame),
+      getCaretPositionOffset(actualFrame)
+    ]), "caret offset");
+  }, "iframe's with equal content should report the same caret offset");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/computed-style-001.html b/testing/web-platform/tests/css/cssom/computed-style-001.html
new file mode 100644
index 0000000000..e487981e30
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/computed-style-001.html
@@ -0,0 +1,66 @@
+<!DOCTYPE html>
+<html>
+ <head>
+  <title>CSS Test: getComputedStyle</title>
+  <link rel="author" title="Bear Travis" href="mailto:betravis@adobe.com">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#extensions-to-the-window-interface">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstyledeclaration-interface">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#resolved-values">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="getComputedStyle returns a readonly CSSStyleDeclaration with resolved values">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+  <style>
+    #outside {
+        width: 200px;
+        height: 200px;
+    }
+    #outside div {
+        font-size: 100px;
+    }
+    #inside {
+        width: 50%;
+        height: 100px;
+    }
+  </style>
+ </head>
+ <body>
+ <noscript>Test not run - javascript required.</noscript>
+ <div id="log"></div>
+ <div id="outside"><div id="inside"></div></div>
+ <script type="text/javascript">
+    var outer = document.getElementById("outside");
+    var inner = document.getElementById("inside");
+    var innerStyle;
+
+    // do not allow modifications to a computed CSSStyleDeclaration
+    test(function() {
+        innerStyle = window.getComputedStyle(inner);
+        assert_throws_dom(  "NO_MODIFICATION_ALLOWED_ERR",
+                        function() { innerStyle.cssText = "color: blue;"; },
+                        "do not allow setting cssText on a readonly CSSStyleDeclaration");
+        assert_throws_dom(  "NO_MODIFICATION_ALLOWED_ERR",
+                        function() { innerStyle.setProperty("color", "blue"); },
+                        "do not allow calling setProperty on a readonly CSSStyleDeclaration");
+        assert_throws_dom(  "NO_MODIFICATION_ALLOWED_ERR",
+                        function() { innerStyle.color = "blue"; },
+                        "do not allow setting a property on a readonly CSSStyleDeclaration");
+    }, "read_only");
+
+    // Directly set properties are resolved
+    test(function() {
+        assert_equals(innerStyle.getPropertyValue("height"), "100px");
+    }, "property_values");
+
+    // Inherited properties are resolved
+    test(function() {
+        assert_equals(innerStyle.getPropertyValue("font-size"), "100px");
+    }, "inherited_property_values");
+
+    // Relative properties are resolved
+    test(function() {
+        assert_equals(innerStyle.getPropertyValue("width"), "100px");
+    }, "relative_property_values");
+ </script>
+ </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/computed-style-002.html b/testing/web-platform/tests/css/cssom/computed-style-002.html
new file mode 100644
index 0000000000..7b31c96c09
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/computed-style-002.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<title>CSS Test: getComputedStyle - resolved width in iframe</title>
+<link rel="author" title="Rune Lillesveen" href="mailto:futhark@chromium.org" />
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-values" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<iframe id="frm" width="100"></iframe>
+<script>
+  test(() => {
+    const frmDoc = frm.contentWindow.document;
+    frmDoc.open();
+    frmDoc.write('<body style="margin:0"><div style="width:100%"></div>');
+    frmDoc.close();
+
+    assert_equals(frm.contentWindow.getComputedStyle(frmDoc.querySelector("div")).width, "100px");
+  }, "Check that a percent width in an iframe is resolved against iframe width for getComputedStyle.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/computed-style-003.html b/testing/web-platform/tests/css/cssom/computed-style-003.html
new file mode 100644
index 0000000000..e73b793b8f
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/computed-style-003.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<title>CSS Test: getComputedStyle - resolved width in iframe dynamic display</title>
+<link rel="author" title="Rune Lillesveen" href="mailto:futhark@chromium.org" />
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-values" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<iframe id="frm" width="100" style="display:none"></iframe>
+<script>
+  const frmDoc = frm.contentWindow.document;
+  frmDoc.open();
+  frmDoc.write('<body style="margin:0"><div style="width:100%"></div>');
+  frmDoc.close();
+
+  document.body.offsetWidth; // Make sure we layout the top document.
+
+  test(() => {
+    frm.style.display = "inline";
+    assert_equals(frm.contentWindow.getComputedStyle(frmDoc.querySelector("div")).width, "100px");
+  }, "Check that a percent width in an iframe is the resolved width when the iframe is displayed.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/computed-style-004.html b/testing/web-platform/tests/css/cssom/computed-style-004.html
new file mode 100644
index 0000000000..c5b08712f9
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/computed-style-004.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<title>CSS Test: getComputedStyle - resolved width in nested iframes dynamic width</title>
+<link rel="author" title="Rune Lillesveen" href="mailto:futhark@chromium.org" />
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-values" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<iframe id="outer" width="100" scrolling="no" frameborder="0"></iframe>
+<script>
+  const outerDoc = outer.contentWindow.document;
+  outerDoc.open();
+  outerDoc.write('<body style="margin:0"><iframe id="inner" scrolling="no" frameborder="0" style="width:100%"></iframe>');
+  outerDoc.close();
+
+  const innerWindow = outerDoc.querySelector("#inner").contentWindow;
+  const innerDoc = innerWindow.document;
+  innerDoc.open();
+  innerDoc.write('<body style="margin:0"><div style="width:100%"></div>');
+  innerDoc.close();
+  innerDoc.body.offsetWidth; // Make sure we layout the top document.
+
+  test(() => {
+    assert_equals(innerWindow.getComputedStyle(innerDoc.querySelector("div")).width, "100px");
+  }, "Check that the initial width is 100px.");
+
+  test(() => {
+    outer.setAttribute("width", "200");
+    assert_equals(innerWindow.getComputedStyle(innerDoc.querySelector("div")).width, "200px");
+  }, "Check that the resolved width of the inner div is affected by changing the width of outer iframe.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/computed-style-005.html b/testing/web-platform/tests/css/cssom/computed-style-005.html
new file mode 100644
index 0000000000..49a5977acd
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/computed-style-005.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<html>
+ <head>
+  <title>CSS Test: getComputedStyle on blocks with auto margins</title>
+  <link rel="author" title="Brad Werth" href="mailto:bwerth@mozilla.com">
+  <link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle" />
+  <meta name="assert" content="getComputedStyle produces pixel values for margin: auto blocks">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+  <style>
+    x {
+      display: block;
+      position: relative;
+      background: grey;
+      width: 60px;
+      height: 10px;
+    }
+    y {
+      display: block;
+      background: blue;
+      width: 40px;
+      height: 10px;
+      margin: auto;
+    }
+
+    #absolute {
+      position: absolute;
+      left: 0;
+      right: 0;
+    }
+    #relative {
+      position: relative;
+    }
+  </style>
+ </head>
+ <body>
+ <div id="log"></div>
+ <x><y id="absolute"></y></x>
+ <x><y id="relative"></y></x>
+ <script type="text/javascript">
+   let idsToTest = [
+     "absolute",
+     "relative",
+   ];
+
+   for (let id of idsToTest) {
+     let elem = document.getElementById(id);
+     let elemStyle = window.getComputedStyle(elem);
+
+     // positioned element's auto margins should be resolved to 10px.
+     test(function() {
+       assert_equals(elemStyle.getPropertyValue("margin-left"), "10px");
+       assert_equals(elemStyle.getPropertyValue("margin-right"), "10px");
+     }, id + "_computed_margins");
+
+     // positioned element should have a left and right of 0px (as authored).
+     test(function() {
+       assert_equals(elemStyle.getPropertyValue("left"), "0px");
+       assert_equals(elemStyle.getPropertyValue("right"), "0px");
+     }, id + "_computed_left_and_right");
+   }
+ </script>
+ </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/computed-style-set-property.html b/testing/web-platform/tests/css/cssom/computed-style-set-property.html
new file mode 100644
index 0000000000..a1f1380dcc
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/computed-style-set-property.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<title>NoModificationAllowedError when mutating read only properties</title>
+<link rel="help" href="https://www.w3.org/TR/cssom-1/#dom-cssstyledeclaration-setpropertyvalue">
+<meta name="assert" content="This test verifies that NoModificationAllowedError is thrown when mutating read only properties" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body></body>
+<script>
+test(function(t) {
+    assert_equals(document.defaultView.getComputedStyle(document.body, null).parentRule, null);
+}, "Computed style parent (should be null)");
+
+test(function(t) {
+    assert_throws_dom("NoModificationAllowedError", function() {
+        document.defaultView.getComputedStyle(document.body, null).color = "blue";
+    });
+}, "Exception thrown when trying to change a computed style declaration via property");
+
+test(function(t) {
+    assert_throws_dom("NoModificationAllowedError", function() {
+        document.defaultView.getComputedStyle(document.body, null).setProperty("color", "blue");
+    });
+}, "Exception thrown when trying to change a computed style declaration via setProperty");
+
+test(function(t) {
+    assert_throws_dom("NoModificationAllowedError", function() {
+        document.defaultView.getComputedStyle(document.body, null).webkitTransition = "";
+    });
+}, "Exception thrown when trying to change a computed style alias via property");
+
+test(function(t) {
+    assert_throws_dom("NoModificationAllowedError", function() {
+        document.defaultView.getComputedStyle(document.body, null).setProperty("webkitTransition", "");
+    });
+}, "Exception thrown when trying to change a computed style alias via setProperty");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/css-style-attr-decl-block.html b/testing/web-platform/tests/css/cssom/css-style-attr-decl-block.html
new file mode 100644
index 0000000000..744b3a744c
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/css-style-attr-decl-block.html
@@ -0,0 +1,148 @@
+<!DOCTYPE html>
+<meta charset="UTF-8">
+<link rel="author" title="Xidorn Quan" href="mailto:me@upsuper.org">
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#css-declaration-blocks">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body>
+<script>
+function createTestElement(style) {
+  let wrapper = document.createElement("div");
+  wrapper.innerHTML = `<div id="test" style="${style}"></div>`;
+  return wrapper.querySelector("#test");
+}
+
+test(function() {
+  let elem = createTestElement("z-index: 10;");
+  assert_equals(elem.style.cssText, "z-index: 10;");
+}, "Style attribute should create CSS declaration block based on its content");
+
+test(function() {
+  let elem = createTestElement("z-index: 20;");
+  let style = elem.style;
+  assert_equals(style.cssText, "z-index: 20;");
+  function assert_css_text(value, action) {
+    assert_equals(style.cssText, value, "CSS declaration block after " + action);
+  }
+  elem.setAttribute("style", "z-index: 21;");
+  assert_css_text("z-index: 21;", "changing the style attribute");
+  elem.removeAttribute("style");
+  assert_css_text("", "removing the style attribute");
+  elem.setAttribute("style", "position: absolute;");
+  assert_css_text("position: absolute;", "adding style attribute again");
+}, "Changes to style attribute should reflect on CSS declaration block");
+
+test(function() {
+  let elem = createTestElement("z-index: 30;");
+  let style = elem.style;
+  assert_equals(style.cssText, "z-index: 30;");
+  function assert_attr(value, action) {
+    assert_equals(elem.getAttribute("style"), value, "style attribute after " + action);
+  }
+  style.setProperty("z-index", "31");
+  assert_attr("z-index: 31;", "changing property in CSS declaration block");
+  style.removeProperty("z-index");
+  assert_attr("", "removing property from CSS declaration block");
+  style.setProperty("position", "absolute");
+  assert_attr("position: absolute;", "adding property to CSS declaration block");
+  style.cssText = "z-index: 32;";
+  assert_attr("z-index: 32;", "changing cssText");
+  style.cssText = "z-index: 33; invalid";
+  assert_attr("z-index: 33;", "changing cssText to a partial invalid value");
+}, "Changes to CSS declaration block should reflect on style attribute");
+
+test(function() {
+  let elem = createTestElement("z-index: 40;");
+  let style = elem.style;
+  assert_equals(style.cssText, "z-index: 40;");
+  // Create an observer for the element.
+  let observer = new MutationObserver(function() {});
+  observer.observe(elem, {attributes: true, attributeOldValue: true});
+  function assert_record_with_old_value(oldValue, action) {
+    let records = observer.takeRecords();
+    assert_equals(records.length, 1, "number of mutation records after " + action);
+    let record = records[0];
+    assert_equals(record.type, "attributes", "mutation type after " + action);
+    assert_equals(record.attributeName, "style", "mutated attribute after " + action);
+    assert_equals(record.oldValue, oldValue, "old value after " + action);
+  }
+  style.setProperty("z-index", "41");
+  assert_record_with_old_value("z-index: 40;", "changing property in CSS declaration block");
+  style.cssText = "z-index: 42;";
+  assert_record_with_old_value("z-index: 41;", "changing cssText");
+  style.cssText = "z-index: 42;";
+  assert_record_with_old_value("z-index: 42;", "changing cssText with the same content");
+  style.removeProperty("z-index");
+  assert_record_with_old_value("z-index: 42;", "removing property from CSS declaration block");
+  // Mutation to shorthand properties should also trigger only one mutation record.
+  style.setProperty("margin", "1px");
+  assert_record_with_old_value("", "adding shorthand property to CSS declaration block");
+  style.removeProperty("margin");
+  assert_record_with_old_value("margin: 1px;", "removing shorthand property from CSS declaration block");
+  // Final sanity check.
+  assert_equals(elem.getAttribute("style"), "");
+}, "Changes to CSS declaration block should queue mutation record for style attribute");
+
+test(function() {
+  let elem = createTestElement("z-index: 50; invalid");
+  let style = elem.style;
+  assert_equals(style.cssText, "z-index: 50;");
+  // Create an observer for the element.
+  let observer = new MutationObserver(function() {});
+  observer.observe(elem, {attributes: true});
+  function assert_no_record(action) {
+    let records = observer.takeRecords();
+    assert_equals(records.length, 0, "expect no record after " + action);
+  }
+  style.setProperty("z-index", "invalid");
+  assert_no_record("setting invalid value to property");
+  // Longhand property.
+  style.removeProperty("position");
+  assert_no_record("removing non-existing longhand property");
+  style.setProperty("position", "");
+  assert_no_record("setting empty string to non-existing longhand property");
+  // Shorthand property.
+  style.removeProperty("margin");
+  assert_no_record("removing non-existing shorthand property");
+  style.setProperty("margin", "");
+  assert_no_record("setting empty string to non-existing shorthand property");
+  // Check that the value really isn't changed.
+  assert_equals(elem.getAttribute("style"), "z-index: 50; invalid",
+                "style attribute after removing non-existing properties");
+}, "Removing non-existing property or setting invalid value on CSS declaration block shouldn't queue mutation record");
+
+test(function() {
+  let elem = createTestElement("background-image: url(./);");
+  let style = elem.style;
+  let base = document.createElement("base");
+  base.href = "/";
+  document.body.appendChild(elem);
+  let originalComputedValue = getComputedStyle(elem).backgroundImage;
+  document.head.appendChild(base);
+  this.add_cleanup(() => {
+    document.head.removeChild(base);
+    document.body.removeChild(elem);
+  });
+  style.setProperty("background-color", "green");
+  assert_equals(getComputedStyle(elem).backgroundImage, originalComputedValue,
+                "getComputedStyle(elem).backgroundImage after setting background-color");
+  style.setProperty("background-image", "url(./)");
+  assert_not_equals(getComputedStyle(elem).backgroundImage, originalComputedValue,
+                    "getComputedStyle(elem).backgroundImage after setting background-image");
+}, "Changes to CSS declaration block after a base URL change");
+
+test(function() {
+  let e1 = document.createElement('div');
+  let e2 = document.createElement('div');
+  document.body.append(e1, e2);
+  this.add_cleanup(() => {
+    e1.remove();
+    e2.remove();
+  });
+  e1.style.cssText = "all:revert;border-bottom-left-radius:1px;";
+  e2.style.cssText = "all:unset;border-bottom-left-radius:1px;";
+  let processed = e1.style.cssText.split(';')
+    .map(x => x.replace(/revert$/, 'unset')).join(';');
+  assert_equals(processed, e2.style.cssText);
+}, "Expansion of all:unset and all:revert treated identically");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/css-style-attribute-modifications.html b/testing/web-platform/tests/css/cssom/css-style-attribute-modifications.html
new file mode 100644
index 0000000000..524a5adca3
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/css-style-attribute-modifications.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:ecobos@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-elementcssinlinestyle-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="test" style="color: red"></div>
+<script>
+test(function() {
+  var el = document.getElementById("test");
+  el.style.color = "";
+  assert_true(el.hasAttribute("style"));
+
+  el.removeAttribute("style");
+  assert_false(el.hasAttribute("style"));
+}, "Mutating the style declaration doesn't remove the style attribute");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/css-style-declaration-modifications.html b/testing/web-platform/tests/css/cssom/css-style-declaration-modifications.html
new file mode 100644
index 0000000000..a0e2275491
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/css-style-declaration-modifications.html
@@ -0,0 +1,71 @@
+<!DOCTYPE html>
+<html>
+ <head>
+  <title>CSS Test: CSSStyleDeclaration Interface</title>
+  <link rel="author" title="Bear Travis" href="mailto:betravis@adobe.com">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstyledeclaration-interface">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="CSSStyleDeclaration is properly initialized and can be modified through its interface">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+  <style id="styleElement">
+      #test { color: green; }
+  </style>
+ </head>
+ <body>
+ <div id="log"></div>
+ <div id="test"></div>
+ <script type="text/javascript">
+    var declaration;
+    setup(function() {
+        var styleElement = document.getElementById("styleElement");
+        declaration = styleElement.sheet.cssRules.item(0).style;
+    });
+
+    test(function() {
+        assert_equals(declaration.cssText, "color: green;");
+        assert_equals(declaration.getPropertyValue("color"), "green");
+    }, "Reading CSSStyleDeclaration initialized from a style element");
+
+    test(function() {
+        declaration.cssText = "margin-left:10px;  padding-left:10px";
+        assert_equals(declaration.cssText, "margin-left: 10px; padding-left: 10px;");
+        assert_equals(declaration.length, 2);
+        assert_equals(declaration.item(0), "margin-left");
+        assert_equals(declaration.item(1), "padding-left");
+        assert_equals(declaration.getPropertyValue("margin-left"), "10px");
+        assert_equals(declaration.getPropertyValue("padding-left"), "10px");
+
+        var computedStyle = window.getComputedStyle(document.getElementById("test"));
+        assert_equals(computedStyle.getPropertyValue("margin-left"), "10px");
+        assert_equals(computedStyle.getPropertyValue("padding-left"), "10px");
+    }, "Setting CSSStyleDeclaration#cssText");
+
+    test(function() {
+        while (declaration.length > 0) {
+            declaration.removeProperty(declaration.item(0));
+        }
+        declaration.setProperty("margin-left", "15px");
+        declaration.setProperty("padding-left", "15px");
+
+
+        assert_equals(declaration.length, 2);
+        assert_equals(declaration.item(0), "margin-left");
+        assert_equals(declaration.item(1), "padding-left");
+        assert_equals(declaration.getPropertyValue("margin-left"), "15px");
+        assert_equals(declaration.getPropertyValue("padding-left"), "15px");
+
+        var computedStyle = window.getComputedStyle(document.getElementById("test"));
+        assert_equals(computedStyle.getPropertyValue("margin-left"), "15px");
+        assert_equals(computedStyle.getPropertyValue("padding-left"), "15px");
+
+    }, "Calling CSSStyleDeclaration#setProperty");
+
+    test(function() {
+        declaration.setProperty("background-color", "red", "ImPoRtAnt");
+        assert_equals(declaration.getPropertyPriority("background-color"), "important");
+    }, "setProperty priority should be case-insensitive");
+
+ </script>
+ </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/css-style-reparse.html b/testing/web-platform/tests/css/cssom/css-style-reparse.html
new file mode 100644
index 0000000000..d5153cf267
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/css-style-reparse.html
@@ -0,0 +1,59 @@
+<!doctype html>
+<html>
+<head>
+  <meta charset=utf-8>
+  <title>CSS Test: DOM modification re-parsing test</title>
+  <link rel="help" href="https://drafts.csswg.org/cssom/">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+  <script src=/resources/testharness.js></script>
+  <script src=/resources/testharnessreport.js></script>
+  <style>div { min-width: 0px; }</style>
+  <style id="style-element"></style>
+</head>
+<body>
+<div id="test-div"></div>
+<script type="text/javascript">
+    var style = document.getElementById("style-element");
+    var div = document.getElementById("test-div");
+
+    function testProperty(prop) {
+      // Assigning an empty string to textContent or innerHTML should trigger a
+      // reparse only if the element is not empty.
+      style.sheet.insertRule("#test-div { min-width: 42px; }");
+      assert_equals(getComputedStyle(div).minWidth, "42px");
+
+      style[prop] = "";
+      assert_equals(getComputedStyle(div).minWidth, "42px");
+
+      style[prop] = " ";
+      assert_equals(getComputedStyle(div).minWidth, "0px");
+
+      style.sheet.insertRule("#test-div { min-width: 42px; }");
+      assert_equals(getComputedStyle(div).minWidth, "42px");
+
+      style[prop] = "";
+      assert_equals(getComputedStyle(div).minWidth, "0px");
+
+      style.sheet.insertRule("#test-div { min-width: 42px; }");
+      assert_equals(getComputedStyle(div).minWidth, "42px");
+
+      style.appendChild(document.createTextNode(""));
+      assert_equals(getComputedStyle(div).minWidth, "0px");
+
+      style.sheet.insertRule("#test-div { min-width: 42px; }");
+      assert_equals(getComputedStyle(div).minWidth, "42px");
+
+      style[prop] = "";
+      assert_equals(getComputedStyle(div).minWidth, "0px");
+    }
+
+    test(function() {
+      testProperty("textContent");
+    }, "style.textContent modification");
+
+    test(function() {
+      testProperty("innerHTML");
+    }, "style.innerHTML modification");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssimportrule-parent.html b/testing/web-platform/tests/css/cssom/cssimportrule-parent.html
new file mode 100644
index 0000000000..2792af69c0
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssimportrule-parent.html
@@ -0,0 +1,20 @@
+<!doctype html>
+<title>CSSImportRule correctly unlinks its child stylesheet from its parent</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-cssimportrule-interface">
+<style>
+  @import "data:text/css,:root{background:red}";
+</style>
+<script>
+let t = async_test("@import stylesheet is properly unlinked from parent after removal");
+window.onload = t.step_func_done(function() {
+  let sheet = document.styleSheets[0];
+  let childSheet = sheet.cssRules[0].styleSheet;
+  assert_not_equals(childSheet, null, "@import rule should have a stylesheet");
+  assert_equals(childSheet.parentStyleSheet, sheet, "@import rule should the correct parent");
+  sheet.deleteRule(0);
+  assert_equals(childSheet.parentStyleSheet, null, "@import rule should be correctly unlinked");
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssimportrule-sheet-identity.html b/testing/web-platform/tests/css/cssom/cssimportrule-sheet-identity.html
new file mode 100644
index 0000000000..3db5634ebe
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssimportrule-sheet-identity.html
@@ -0,0 +1,24 @@
+<!doctype html>
+<title>CSSImportRule has different sheets even if referencing the same URL</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-cssimportrule-interface">
+<link rel="stylesheet" href="support/import-rule.css">
+<link rel="stylesheet" href="support/import-rule.css">
+<script>
+let t = async_test("CSSImportRule has different sheets even if referencing the same URL");
+window.onload = t.step_func_done(function() {
+  let sheet1 = document.styleSheets[0];
+  let sheet2 = document.styleSheets[1];
+
+  assert_not_equals(sheet1, sheet2);
+
+  let childSheet1 = sheet1.cssRules[0].styleSheet;
+  let childSheet2 = sheet2.cssRules[0].styleSheet;
+
+  assert_not_equals(childSheet1, null);
+  assert_not_equals(childSheet2, null);
+  assert_not_equals(childSheet1, childSheet2, "@import pointing to the same URL shouldn't point to the same StyleSheet object");
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssimportrule.html b/testing/web-platform/tests/css/cssom/cssimportrule.html
new file mode 100644
index 0000000000..2703b9a69b
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssimportrule.html
@@ -0,0 +1,122 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSSOM CSSRule CSSImportRule interface</title>
+    <link rel="author" title="Letitia Lew" href="mailto:lew.letitia@gmail.com">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#css-rules">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssimportrule-interface">
+    <meta name="flags" content="dom">
+    <meta name="assert" content="All properties for this CSSImportRule instance of CSSRule are initialized correctly">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+
+    <style id="styleElement" type="text/css">
+        @import url("support/a-green.css");
+        @import url("support/a-green.css") screen;
+        @import url("support/a-green.css") all;
+        @import url("support/a-green") supports((display: flex) or (display: block));
+        @page { background-color: red; }
+    </style>
+</head>
+<body>
+    <div id="log"></div>
+
+    <script type="text/javascript">
+        var styleSheet, ruleList, rule, ruleWithMedia, ruleWithMediaAll, ruleWithSupports;
+        setup(function() {
+            styleSheet = document.getElementById("styleElement").sheet;
+            ruleList = styleSheet.cssRules;
+            rule = ruleList[0];
+            ruleWithMedia = ruleList[1];
+            ruleWithMediaAll = ruleList[2];
+            ruleWithSupports = ruleList[3];
+        });
+
+        test(function() {
+            assert_true(rule instanceof CSSRule);
+            assert_true(rule instanceof CSSImportRule);
+            assert_true(ruleWithMedia instanceof CSSRule);
+            assert_true(ruleWithMedia instanceof CSSImportRule);
+            assert_true(ruleWithSupports instanceof CSSRule);
+            assert_true(ruleWithSupports instanceof CSSImportRule);
+        }, "CSSRule and CSSImportRule types");
+
+        test(function() {
+            assert_equals(rule.STYLE_RULE, 1);
+            assert_equals(rule.IMPORT_RULE, 3);
+            assert_equals(rule.MEDIA_RULE, 4);
+            assert_equals(rule.FONT_FACE_RULE, 5);
+            assert_equals(rule.PAGE_RULE, 6);
+            assert_equals(rule.NAMESPACE_RULE, 10);
+            assert_idl_attribute(rule, "type");
+            assert_equals(typeof rule.type, "number");
+        }, "Type of CSSRule#type and constant values");
+
+        test(function() {
+            assert_true(rule instanceof CSSRule);
+            assert_idl_attribute(rule, "cssText");
+            assert_idl_attribute(rule, "parentRule");
+            assert_idl_attribute(rule, "parentStyleSheet");
+
+            assert_readonly(rule, "type");
+            assert_readonly(rule, "parentRule");
+            assert_readonly(rule, "parentStyleSheet");
+        }, "Existence and writability of CSSRule attributes");
+
+        test(function() {
+            assert_equals(rule.type, rule.IMPORT_RULE);
+            assert_equals(typeof rule.cssText, "string");
+            assert_equals(rule.cssText, '@import url("support/a-green.css");');
+            assert_equals(ruleWithMedia.cssText, '@import url("support/a-green.css") screen;');
+            assert_equals(ruleWithMediaAll.cssText, '@import url("support/a-green.css") all;');
+            assert_equals(ruleWithSupports.cssText, '@import url("support/a-green") supports((display: flex) or (display: block));');
+            assert_equals(rule.parentRule, null);
+            assert_true(rule.parentStyleSheet instanceof CSSStyleSheet);
+        }, "Values of CSSRule attributes");
+
+        test(function() {
+            assert_idl_attribute(rule, "href");
+            assert_idl_attribute(rule, "media");
+            assert_idl_attribute(rule, "styleSheet");
+
+            assert_readonly(rule, "href");
+            assert_readonly(rule, "media");
+            assert_readonly(rule, "styleSheet");
+        }, "Existence and writability of CSSImportRule attributes");
+
+        test(function() {
+            assert_equals(typeof rule.href, "string");
+            assert_true(rule.media instanceof MediaList);
+            assert_true(rule.styleSheet instanceof CSSStyleSheet);
+            assert_true(ruleWithMedia.media.length > 0);
+            assert_equals(ruleWithMedia.media.mediaText, "screen");
+        }, "Values of CSSImportRule attributes");
+
+        test(function() {
+            ruleWithMedia.media = "print";
+            assert_equals(ruleWithMedia.media.mediaText, "print");
+        }, "CSSImportRule : MediaList mediaText attribute should be updated due to [PutForwards]");
+
+        test(function() {
+            var ruleWithPage = ruleList[4];
+            ruleWithPage.style = "margin-top: 10px;"
+            assert_equals(ruleWithPage.style.cssText, "margin-top: 10px;");
+        }, "CSSStyleDeclaration cssText attribute should be updated due to [PutForwards]");
+
+        test(function() {
+            styleSheet.media = "screen";
+            assert_equals(styleSheet.media.mediaText, "screen");
+        }, "StyleSheet : MediaList mediaText attribute should be updated due to [PutForwards]");
+
+        test(function() {
+            assert_idl_attribute(ruleWithSupports, "supportsText");
+            assert_readonly(ruleWithSupports, "supportsText");
+        }, "Existence and writability of CSSImportRule supportsText attribute");
+
+        test(function() {
+            assert_equals(ruleWithSupports.supportsText, "(display: flex) or (display: block)");
+        }, "Value of CSSImportRule supportsText attribute");
+    </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssom-cssText-serialize.html b/testing/web-platform/tests/css/cssom/cssom-cssText-serialize.html
new file mode 100644
index 0000000000..27479f026d
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-cssText-serialize.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM Parsing Test: getting cssText must return the result of serializing the CSS declaration blocks.</title>
+        <link rel="author" title="Paul Irish" href="mailto:paul.irish@gmail.com">
+        <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstyledeclaration-interface">
+
+        <link rel="source" href="http://trac.webkit.org/export/120528/trunk/LayoutTests/fast/css/cssText-cache.html">
+        <meta name="flags" content="dom">
+
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+    </head>
+
+    <body>
+        <div id="log"></div>
+        <div id="box"></div>
+        <script>
+          test(function() {
+            var style = document.getElementById('box').style;
+            style.left = "10px";
+            assert_equals(style.cssText, "left: 10px;");
+            style.right = "20px";
+            assert_equals(style.cssText, "left: 10px; right: 20px;");
+          }, 'CSSStyleDeclaration cssText serializes declaration blocks.');
+        </script>
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssom-cssstyledeclaration-set.html b/testing/web-platform/tests/css/cssom/cssom-cssstyledeclaration-set.html
new file mode 100644
index 0000000000..1fb7ccd324
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-cssstyledeclaration-set.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM: CSSStyleDeclaration on HTMLElement represents inline style changes</title>
+        <link rel="author" title="Paul Irish" href="mailto:paul.irish@gmail.com">
+        <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstyledeclaration-interface">
+
+        <link rel="source" href="http://trac.webkit.org/export/120528/trunk/LayoutTests/fast/css/cssText-cache.html">
+        <meta name="flags" content="dom">
+
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+    </head>
+
+    <body>
+        <div id="log"></div>
+
+        <div id="box"></div>
+
+        <script>
+
+          var style = document.getElementById('box').style;
+
+          test(function(){
+
+            style.left = "10px";
+            assert_equals(style.left, "10px", 'left property set on element\'s CSSStyleDeclaration Object');
+            style.left = "20px";
+            assert_equals(style.left, "20px", 'left property set on element\'s CSSStyleDeclaration Object');
+
+          }, 'CSSStyleDeclaration on HTMLElement represents inline style changes');
+
+        </script>
+
+
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssom-fontfacerule-constructors.html b/testing/web-platform/tests/css/cssom/cssom-fontfacerule-constructors.html
new file mode 100644
index 0000000000..ed1cab257d
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-fontfacerule-constructors.html
@@ -0,0 +1,63 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM Parsing Test: @font-face rules toString() as valid interfaces</title>
+        <link rel="author" title="Paul Irish" href="mailto:paul.irish@gmail.com">
+        <link rel="reviewer" title="Ms2ger" href="mailto:ms2ger@gmail.com"> <!-- 2012-06-17 -->
+        <link rel="help" href="https://drafts.csswg.org/css-fonts-4/#om-fontface">
+
+        <meta name="flags" content="dom">
+
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+    </head>
+
+    <body>
+        <div id="log"></div>
+
+
+        <style id="teststyles">
+        @font-face {
+            src: url(http://foo/bar/font.ttf);
+        }
+        @font-face {
+            font-family: STIXGeneral;
+            src: local(STIXGeneral), url(/stixfonts/STIXGeneral.otf);
+            unicode-range: U+000-49F, U+2000-27FF, U+2900-2BFF, U+1D400-1D7FF;
+        }
+        @font-face {
+            font-family: MainText;
+            src: url(http://example.com/font.ttf);
+            font-variant: oldstyle-nums proportional-nums styleset(1,3);
+        }
+
+       @font-face {
+            font-family: BodyText;
+            src: local("HiraMaruPro-W4");
+            font-variant: proportional-width;
+            font-feature-settings: "ital"; /* Latin italics within CJK text feature */
+        }
+        </style>
+
+
+        <script>
+            var validRules = document.getElementById('teststyles').sheet.cssRules;
+
+            test(function(){
+                for (var i = 0; i < validRules.length; ++i) {
+                    assert_equals(validRules.item(i).toString(), '[object CSSFontFaceRule]');
+                }
+            }, '@font-face declarations are instances of CSSFontFaceRule')
+
+
+            test(function(){
+                for (var i = 0; i < validRules.length; ++i) {
+                    assert_equals(validRules.item(i).style.toString(), '[object CSSStyleDeclaration]');
+                }
+            }, 'The style attribute must return a CSSStyleDeclaration block')
+
+        </script>
+
+
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssom-fontfacerule.html b/testing/web-platform/tests/css/cssom/cssom-fontfacerule.html
new file mode 100644
index 0000000000..9a06808e3a
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-fontfacerule.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM Parsing Test: @font-face rules parsed into CSSOM CSSFontFaceRules</title>
+        <link rel="author" title="Paul Irish" href="mailto:paul.irish@gmail.com">
+        <link rel="help" href="https://drafts.csswg.org/css-fonts-4/#om-fontface">
+
+        <meta name="flags" content="dom">
+
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+    </head>
+
+    <body>
+        <div id="log"></div>
+
+
+        <style id="teststyles">
+        @font-face {
+            src: url(http://foo/bar/font.ttf);
+        }
+        @font-face {
+            font-family: STIXGeneral;
+            src: local(STIXGeneral), url(/stixfonts/STIXGeneral.otf);
+            unicode-range: U+000-49F, U+2000-27FF, U+2900-2BFF, U+1D400-1D7FF;
+        }
+        @font-face {
+            font-family: MainText;
+            src: url(http://example.com/font.ttf);
+            font-variant: oldstyle-nums proportional-nums styleset(1,3);
+        }
+
+       @font-face {
+            font-family: BodyText;
+            src: local("HiraMaruPro-W4");
+            font-variant: proportional-width;
+            font-feature-settings: "ital"; /* Latin italics within CJK text feature */
+        }
+        </style>
+
+
+        <script>
+            var validRules = document.getElementById('teststyles').sheet.cssRules;
+
+            test(function(){
+
+                assert_equals(validRules[0].style.src, 'url("http://foo/bar/font.ttf")');
+                assert_equals(validRules[1].style.fontFamily, 'STIXGeneral');
+
+                /* unimplemented @font-face properties are not represented in CSSOM */
+
+            }, 'CSSStyleDeclaration values are represented within CSSFontFaceRule')
+
+        </script>
+
+
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssom-getPropertyValue-common-checks.html b/testing/web-platform/tests/css/cssom/cssom-getPropertyValue-common-checks.html
new file mode 100644
index 0000000000..27d802cd3d
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-getPropertyValue-common-checks.html
@@ -0,0 +1,223 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Common serialization checks for all properties</title>
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com" />
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-getpropertyvalue">
+
+<div id="element"></div>
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+const element = document.getElementById("element");
+const { style } = element;
+const computedStyle = getComputedStyle(element);
+const cssProperties = new Set();
+const cssShorthands = new Map();
+const cssShorthandsForLonghand = new Map();
+const cssLonghands = new Set();
+const cssAliases = new Map();
+const initialValues = new Map();
+
+setup(function() {
+  for (let obj = style; obj; obj = Reflect.getPrototypeOf(obj)) {
+    for (let name of Object.getOwnPropertyNames(obj)) {
+      const property = name.replace(/[A-Z]/g, c => "-" + c.toLowerCase());
+      if (CSS.supports(property, "initial")) {
+        cssProperties.add(property);
+      }
+    }
+  }
+  for (let property of cssProperties) {
+    style.cssText = "";
+    style.setProperty(property, "initial");
+    if (style.length > 1) {
+      cssShorthands.set(property, [...style]);
+      for (let longhand of style) {
+        if (cssShorthandsForLonghand.has(longhand)) {
+          cssShorthandsForLonghand.get(longhand).add(property);
+        } else {
+          cssShorthandsForLonghand.set(longhand, new Set([property]));
+        }
+      }
+    } else if (style.length === 1) {
+      if (property === style[0]) {
+        cssLonghands.add(property);
+      } else {
+        cssAliases.set(property, style[0]);
+      }
+    }
+  }
+});
+
+test(function() {
+  const bad = [];
+  for (let property of cssProperties) {
+    style.cssText = "";
+    style.setProperty(property, "initial");
+    const result = style.getPropertyValue(property);
+    if (result !== "initial") {
+      bad.push([property, result]);
+    }
+  }
+  assert_array_equals(bad, []);
+}, "All properties can serialize 'initial'");
+
+test(function() {
+  for (let longhand of cssLonghands) {
+    element.style.setProperty(longhand, "initial");
+  }
+  const bad = [];
+  for (let property of cssProperties) {
+    const result = computedStyle.getPropertyValue(property);
+    if (CSS.supports(property, result)) {
+      initialValues.set(property, result);
+    } else if (property !== "all") {
+      bad.push([property, result]);
+    }
+  }
+  assert_array_equals(bad, []);
+}, "All properties (except 'all') can serialize their initial value (computed)");
+
+test(function() {
+  const bad = [];
+  for (let [property, value] of initialValues) {
+    style.cssText = "";
+    style.setProperty(property, value);
+    const result = style.getPropertyValue(property);
+    if (!CSS.supports(property, result) && property !== "all") {
+      bad.push([property, value, result]);
+    }
+  }
+  assert_array_equals(bad, []);
+}, "All properties (except 'all') can serialize their initial value (specified)");
+
+test(function() {
+  const bad = [];
+  for (let [shorthand, longhands] of cssShorthands) {
+    style.cssText = "";
+    for (let longhand of longhands) {
+      style.setProperty(longhand, "initial");
+    }
+    const result = style.getPropertyValue(shorthand);
+    if (result !== "initial") {
+      bad.push([shorthand, result]);
+    }
+  }
+  assert_array_equals(bad, []);
+}, "All shorthands can serialize their longhands set to 'initial'");
+
+test(function() {
+  const bad = [];
+  outerloop:
+  for (let [shorthand, longhands] of cssShorthands) {
+    style.cssText = "";
+    for (let longhand of longhands) {
+      if (!initialValues.has(longhand)) {
+        continue outerloop;
+      }
+      style.setProperty(longhand, initialValues.get(longhand));
+    }
+    const result = style.getPropertyValue(shorthand);
+    if (!CSS.supports(shorthand, result) && shorthand !== "all") {
+      bad.push([shorthand, result]);
+    }
+  }
+  assert_array_equals(bad, []);
+}, "All shorthands (except 'all') can serialize their longhands set to their initial value");
+
+test(function() {
+  const bad = [];
+  for (let [alias, target] of cssAliases) {
+    style.cssText = "";
+    style.setProperty(target, "initial");
+    const result = style.getPropertyValue(alias);
+    if (result !== "initial") {
+      bad.push([alias, result]);
+    }
+  }
+  assert_array_equals(bad, []);
+}, "All aliases can serialize target property set to 'initial'");
+
+test(function() {
+  const bad = [];
+  for (let [alias, target] of cssAliases) {
+    if (!initialValues.has(target)) {
+      continue;
+    }
+    style.cssText = "";
+    style.setProperty(target, initialValues.get(target));
+    const result = style.getPropertyValue(alias);
+    if (!CSS.supports(alias, result)) {
+      bad.push([alias, result]);
+    }
+  }
+  assert_array_equals(bad, []);
+}, "All aliases can serialize target property set to its initial value");
+
+test(function() {
+  const bad = [];
+  for (let [shorthand, longhands] of cssShorthands) {
+    for (let longhand of longhands) {
+      style.cssText = "";
+      style.setProperty(shorthand, "initial");
+      style.setProperty(longhand, "inherit");
+      const result = style.getPropertyValue(shorthand);
+      if (result !== "") {
+        bad.push([shorthand, longhand, result]);
+      }
+    }
+  }
+  assert_array_equals(bad, []);
+}, "Can't serialize shorthand when longhands are set to different css-wide keywords");
+
+test(function() {
+  const bad = [];
+  for (let [shorthand, longhands] of cssShorthands) {
+    for (let longhand of longhands) {
+      style.cssText = "";
+      style.setProperty(shorthand, "initial");
+      style.setProperty(longhand, "initial", "important");
+      const result = style.getPropertyValue(shorthand);
+      if (result !== "") {
+        bad.push([shorthand, longhand, result]);
+      }
+    }
+  }
+  assert_array_equals(bad, []);
+}, "Can't serialize shorthand when longhands have different priority");
+
+test(function() {
+  const bad = [];
+  for (let [shorthand, longhands] of cssShorthands) {
+    for (let longhand of longhands) {
+      style.cssText = "";
+      style.setProperty(shorthand, "initial");
+      style.removeProperty(longhand);
+      const result = style.getPropertyValue(shorthand);
+      if (result !== "") {
+        bad.push([shorthand, longhand, result]);
+      }
+    }
+  }
+  assert_array_equals(bad, []);
+}, "Can't serialize shorthand set to 'initial' when some longhand is missing");
+
+test(function() {
+  const bad = [];
+  for (let [shorthand, longhands] of cssShorthands) {
+    if (initialValues.has(shorthand)) {
+      for (let longhand of longhands) {
+        style.cssText = "";
+        style.setProperty(shorthand, initialValues.get(shorthand));
+        style.removeProperty(longhand);
+        const result = style.getPropertyValue(shorthand);
+        if (result !== "") {
+          bad.push([shorthand, longhand, result]);
+        }
+      }
+    }
+  }
+  assert_array_equals(bad, []);
+}, "Can't serialize shorthand set to initial value when some longhand is missing");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssom-pagerule.html b/testing/web-platform/tests/css/cssom/cssom-pagerule.html
new file mode 100644
index 0000000000..c7604eba33
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-pagerule.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<title>CSSOM: CSSPageRule tests</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-csspagerule-interface" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  @page {}
+</style>
+<script>
+  const sheet = document.styleSheets[0];
+  const rule = sheet.cssRules[0];
+
+  test(() => {
+    assert_true(!!rule);
+    assert_equals(rule.type, CSSRule.PAGE_RULE);
+  }, "Sanity checks");
+
+  test(() => {
+    assert_equals(rule.selectorText, "");
+  }, "Page selector is initially the empty string");
+
+  test(() => {
+    rule.selectorText = ":left";
+    assert_equals(rule.selectorText, ":left");
+  }, "Set selectorText to :left pseudo page");
+
+  test(() => {
+    rule.selectorText = "named";
+    assert_equals(rule.selectorText, "named");
+  }, "Set selectorText to named page");
+
+  test(() => {
+    rule.selectorText = "named:first";
+    assert_equals(rule.selectorText, "named:first");
+  }, "Set selectorText to named page with :first pseudo page");
+
+  test(() => {
+    assert_equals(rule.parentStyleSheet, sheet);
+    sheet.deleteRule(0);
+    assert_equals(rule.parentStyleSheet, null);
+    rule.selectorText = "pagename";
+  }, "Set selectorText to named page after rule was removed");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssom-ruleTypeAndOrder.html b/testing/web-platform/tests/css/cssom/cssom-ruleTypeAndOrder.html
new file mode 100644
index 0000000000..19aab35357
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-ruleTypeAndOrder.html
@@ -0,0 +1,75 @@
+<!doctype html>
+<head>
+  <title>CSS OM: CSS Rule Types and Order</title>
+  <link rel="help" href="https://drafts.csswg.org/cssom-1/#the-cssrule-interface">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="Testing Serialization of Style Rules">
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+  <style id="s-0">
+    @namespace svg "http://www.w3.org/2000/svg";
+  </style>
+  <style id="s-1">
+    @import url('main.css');
+  </style>
+  <style id="s-2">
+    h1 { background: indianred; }
+  </style>
+  <style id="s-3">
+    @namespace svg "http://www.w3.org/2000/svg";
+    svg|a { color: white; }
+  </style>
+  <style id="s-4">
+    @font-face {
+      font-family: 'Megalopolis';
+      src: url('fonts/megalopolisextra-webfont.eot');
+      src: url('fonts/megalopolisextra-webfont.eot?#iefix') format('embedded-opentype'),
+           url('fonts/megalopolisextra-webfont.woff') format('woff'),
+           url('fonts/megalopolisextra-webfont.ttf') format('truetype'),
+           url('fonts/megalopolisextra-webfont.svg#MEgalopolisExtraRegular') format('svg');
+      font-weight: normal;
+      font-style: normal;
+    }
+  </style>
+  <style id="s-5">
+    @media (min-width: 200px) {
+        h1 { background: aliceblue; }
+    }
+  </style>
+  <style id="s-6">
+    @page :first {
+        h1 { color: #444; }
+    }
+  </style>
+</head>
+<body>
+  <div id="log"></div>
+  <script type="text/javascript">
+    "use strict";
+
+    var stylesheets = document.styleSheets;
+
+    var expectedOrderOfTypes = [10, 3, 1, 1, 5, 4, 6];
+
+    var typesText = {
+      1: 'style rule',
+      3: '@import rule',
+      4: 'media rule',
+      5: '@font-face rule',
+      6: 'page rule',
+      10: 'namespace rule'
+    };
+
+    for (var i = 0; i < stylesheets.length; i++) {
+      test( function () {
+          if (i === 3) {
+            var cssType = stylesheets[i].cssRules[1].type;
+          } else {
+            var cssType = stylesheets[i].cssRules[0].type;
+          }
+          assert_equals(typesText[cssType], typesText[expectedOrderOfTypes[i]]);
+      }, 'Type of #s-' + i + ' is expected to be ' + typesText[expectedOrderOfTypes[i]]);
+    }
+  </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssom-setProperty-shorthand.html b/testing/web-platform/tests/css/cssom/cssom-setProperty-shorthand.html
new file mode 100644
index 0000000000..4c0f9a2f93
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssom-setProperty-shorthand.html
@@ -0,0 +1,77 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM: CSSStyleDeclaration (set|remove)PropertyValue sets/removes shorthand properties</title>
+        <link rel="author" title="Paul Irish" href="mailto:paul.irish@gmail.com">
+        <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstyledeclaration-interface">
+
+        <link rel="source" href="http://trac.webkit.org/export/120528/trunk/LayoutTests/fast/css/cssom-remove-shorthand-property.html">
+        <meta name="flags" content="dom">
+
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+    </head>
+
+    <body>
+        <div id="log"></div>
+
+        <div id="box"></div>
+
+        <script>
+            shorthandProperties = [
+                "font",
+                "border-top",
+                "border-right",
+                "border-bottom",
+                "border-left",
+                "border",
+                "border-color",
+                "border-style",
+                "border-width",
+                "background-position",
+                "background-repeat",
+                "border-spacing",
+                "list-style",
+                "margin",
+                "outline",
+                "padding",
+                "background",
+                "overflow",
+                "border-radius"
+            ];
+
+            element = document.createElement('span');
+
+            function canSetProperty(propertyName, priority) {
+                element.style.setProperty(propertyName, 'initial', priority);
+                return element.style.getPropertyValue(propertyName) == 'initial';
+            }
+
+            function canRemoveProperty(propertyName) {
+                element.style.removeProperty(propertyName);
+                return element.style.getPropertyValue(propertyName) != 'initial';
+            }
+
+            for (i = 0; i < shorthandProperties.length; ++i) {
+                var propertyName = shorthandProperties[i];
+
+                test(function(){
+                    assert_true(canSetProperty(propertyName, ''), 'can setPropertyValue with shorthand');
+                }, 'shorthand ' + propertyName + ' can be set with setProperty');
+
+                test(function(){
+                    assert_true(canRemoveProperty(propertyName), 'can setPropertyValue with shorthand');
+                }, 'shorthand ' + propertyName + ' can be removed with removeProperty');
+
+                test(function(){
+                    assert_true(canSetProperty(propertyName, 'important'), 'can setPropertyValue with shorthand');
+                }, 'shorthand ' + propertyName + ' can be set with setProperty and priority !important');
+
+                test(function(){
+                    assert_true(canRemoveProperty(propertyName), 'can setPropertyValue with shorthand');
+                }, 'shorthand ' + propertyName + ' can be removed with removeProperty even when set with !important');
+
+            }
+        </script>
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-cssfontrule.tentative.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-cssfontrule.tentative.html
new file mode 100644
index 0000000000..2efab276c5
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-cssfontrule.tentative.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSStyleDeclaration for a CSSFontRule</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface">
+<link rel="help" href="https://drafts.csswg.org/css-fonts-4/#cssfontfacerule">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/5649#issuecomment-755796005">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+@font-face {}
+</style>
+<script>
+
+const fontFaceRule = document.styleSheets[0].cssRules[0];
+
+test(() => {
+  assert_true("unicode-range" in fontFaceRule.style);
+  assert_idl_attribute(fontFaceRule.style, "unicode-range");
+}, "a CSSStyleDeclaration for a CSSFontRule contains a unicode-range attribute");
+
+
+test(() => {
+  assert_true("flex-direction" in fontFaceRule.style);
+  assert_idl_attribute(fontFaceRule.style, "flex-direction");
+}, "a CSSStyleDeclaration for a CSSFontRule contains a flex-direction attribute");
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-all-shorthand.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-all-shorthand.html
new file mode 100644
index 0000000000..6619538cf1
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-all-shorthand.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<title>CSSOM test: serialization of the 'all' shorthand in cssText</title>
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#dom-cssstyledeclaration-csstext">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+const style = document.createElement("div").style;
+
+test(function() {
+  style.cssText = "all: inherit";
+  assert_equals(style.cssText, "all: inherit;");
+}, "'all' shorthand alone");
+
+test(function() {
+  style.cssText = "width: 100px; all: inherit; height: inherit";
+  assert_equals(style.cssText, "all: inherit;");
+}, "'all' shorthand with 'width' and 'height'");
+
+test(function() {
+  style.cssText = "direction: ltr; all: inherit; unicode-bidi: plaintext";
+  assert_equals(style.cssText, "direction: ltr; all: inherit; unicode-bidi: plaintext;");
+}, "'all' shorthand with 'direction' and 'unicode-bidi'");
+
+test(function() {
+  style.cssText = "width: 100px; --a: a; all: inherit; --b: b; height: inherit";
+  assert_equals(style.cssText, "--a: a; all: inherit; --b: b;");
+}, "'all' shorthand with 'width', 'height' and custom properties");
+
+test(function() {
+  let cssText = "all: inherit; ";
+  for (let longhand of getComputedStyle(document.documentElement)) {
+    cssText += longhand + ": inherit; ";
+  }
+  style.cssText = cssText;
+  assert_equals(style.cssText, "all: inherit; direction: inherit; unicode-bidi: inherit;");
+}, "'all' shorthand with all longhands");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-final-delimiter.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-final-delimiter.html
new file mode 100644
index 0000000000..01b0a32c3f
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-final-delimiter.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>CSSOM - CSSStyleDeclaration - Text - Serialization - Delimiters</title>
+    <link rel="author" title="Glenn Adams" href="mailto:glenn@skynav.com"/>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface"/>
+    <meta name="flags" content="dom"/>
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+  </head>
+  <body>
+    <div id="box"></div>
+    <script>
+      var style = document.getElementById('box').style;
+
+      test(function(){
+        style.cssText = "";
+        assert_equals(style.cssText, "");
+      }, 'inline style - text - delimiters - zero declarations');
+
+      test(function(){
+        style.cssText = "left: 10px";
+        assert_equals(style.cssText, "left: 10px;");
+      }, 'inline style - text - delimiters - one declaration');
+
+      test(function(){
+        style.cssText = "left: 10px; right: 20px";
+        assert_equals(style.cssText, "left: 10px; right: 20px;");
+      }, 'inline style - text - delimiters - two declarations');
+    </script>
+  </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-important.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-important.html
new file mode 100644
index 0000000000..c12faf63a4
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext-important.html
@@ -0,0 +1,12 @@
+<title>CSSOM test: setting a property with cssText and !important</title>
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#dom-cssstyledeclaration-csstext">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="box"></div>
+<script>
+  var style = document.getElementById('box').style;
+  test(function(){
+      style.cssText = "padding: 10px !important; padding-left: 20px;";
+      assert_equals(style.getPropertyValue("padding-left"), "10px");
+  }, "padding-left should be taken from the !important property");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext.html
new file mode 100644
index 0000000000..686684e614
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-csstext.html
@@ -0,0 +1,126 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM Test: CSSStyleDeclaration.cssText Test</title>
+        <link rel="author" title="kkoichi" href="mailto:coarse.ground@gmail.com">
+        <link rel="reviewer" title="Simon Pieters" href="mailto:simonp@opera.com"><!-- 06-27-2013 -->
+        <link rel="help" href="https://drafts.csswg.org/cssom-1/#dom-cssstyledeclaration-csstext">
+        <meta name="assert" content="CSS declarations is serialized as expected">
+        <meta name="flags" content="dom">
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+    </head>
+    <body>
+    <div id="log"></div>
+    <script>
+        function newElm() {
+            return document.body.appendChild(document.createElement('div'));
+        }
+
+        test(function(){
+            var style = newElm().style;
+            style.COLOR = 'red';
+
+            assert_equals(style.cssText, '');
+
+        }, 'uppercase property');
+
+        test(function(){
+            var style = newElm().style;
+            style.color = 'RED';
+
+            // https://www.w3.org/Bugs/Public/show_bug.cgi?id=29317
+            assert_any(assert_equals, style.cssText, ['color: red;', 'color: RED;']);
+
+        }, 'uppercase value');
+
+        test(function(){
+            var style = newElm().style;
+
+            style.color = 'red';
+
+            style.color = 'unknown color';
+
+            assert_equals(style.cssText, 'color: red;');
+
+        }, 'overwriting with invalid value');
+
+        test(function(){
+            var style = newElm().style;
+            style.color = 'rgb(255, 0, 0)';
+
+            assert_equals(style.cssText, 'color: rgb(255, 0, 0);');
+
+        }, 'use rgb');
+
+        test(function(){
+            var e = newElm();
+            var style = e.style;
+
+            style.color = 'red';
+            style.fontSize = '10pt';
+            style.fontWeight = 'bold';
+
+            assert_equals(style.cssText, 'color: red; font-size: 10pt; font-weight: bold;');
+
+        }, 'cssText order');
+
+        test(function(){
+            var e = newElm();
+            var style = e.style;
+
+            style.fontWeight = 'bold';
+            style.color = 'red';
+            style.fontSize = '10pt';
+
+            assert_equals(style.cssText, 'font-weight: bold; color: red; font-size: 10pt;');
+
+        }, 'another cssText order (non-alphabetical order)');
+
+        test(function(){
+            var style = newElm().style;
+
+            style.color = '   red';
+            style.fontSize = '10pt   ';
+
+            assert_equals(style.cssText, 'color: red; font-size: 10pt;');
+
+        }, 'whitespaces in value');
+
+        test(function(){
+            var style = newElm().style;
+
+            style.color = 'red';
+            style.unknown = 'unknown';
+            style.fontSize = '10pt';
+            assert_equals(style.cssText, 'color: red; font-size: 10pt;');
+
+        }, 'invalid property does not appear');
+
+        test(function(){
+            var style = newElm().style;
+            style.cssText = "margin: 10px; margin-inline: 10px; margin-block: 10px; margin-inline-end: 10px; margin-bottom: 10px;";
+            assert_equals(style.cssText, "margin-top: 10px; margin-right: 10px; margin-left: 10px; margin-inline-start: 10px; margin-block: 10px; margin-inline-end: 10px; margin-bottom: 10px;");
+        }, 'Shorthands aren\'t serialized if there are other properties with different logical groups in between');
+
+        test(function(){
+            var style = newElm().style;
+            style.cssText = "margin-top: 10px; margin-left: 10px; margin-right: 10px; margin-bottom: 10px; margin-inline-start: 10px; margin-inline-end: 10px; margin-block-start: 10px; margin-block-end: 10px;";
+            assert_equals(style.cssText, "margin: 10px; margin-inline: 10px; margin-block: 10px;");
+        }, 'Shorthands _are_ serialized if there are no other properties with different logical groups in between');
+
+        // https://github.com/w3c/csswg-drafts/issues/1033
+        test(function() {
+            var elm = newElm();
+            var style = elm.style;
+
+            style.color = 'red';
+            style.unknown = 'unknown';
+            style.fontSize = '10pt';
+
+            assert_not_equals(getComputedStyle(elm).length, 0, "Should have a style");
+            assert_equals(getComputedStyle(elm).cssText, "", "cssText is empty");
+        }, 'cssText on computed style declaration returns the empty string');
+    </script>
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-custom-properties.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-custom-properties.html
new file mode 100644
index 0000000000..92cf2c847d
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-custom-properties.html
@@ -0,0 +1,19 @@
+<!doctype html>
+<title>CSS Test: computed style declaration includes custom properties.</title>
+<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel="author" href="https://mozilla.org" title="Mozilla">
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/1316">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div style="--foo:bar"></div>
+<div></div>
+<script>
+  test(function() {
+    let withCustom = getComputedStyle(document.querySelector("div"));
+    let withoutCustom = getComputedStyle(document.querySelector("div + div"));
+    assert_equals(withCustom.getPropertyValue("--foo"), "bar", "Should be returned from getPropertyValue");
+    assert_equals(withCustom.length, withoutCustom.length + 1, "Should show up in .length");
+    assert_equals(withCustom[withCustom.length - 1], "--foo", "Should be after all the non-custom properties");
+  }, "Custom properties are included in computed style");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutability.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutability.html
new file mode 100644
index 0000000000..ce5b598ca6
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutability.html
@@ -0,0 +1,69 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM: CSSStyleDeclaration is mutable and immutable in various settings</title>
+        <link rel="author" title="Paul Irish" href="mailto:paul.irish@gmail.com">
+        <link rel="reviewer" title="Ms2ger" href="mailto:ms2ger@gmail.com"> <!-- 2012-06-17 -->
+        <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstyledeclaration-interface">
+
+        <meta name="flags" content="dom">
+
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+    </head>
+
+    <body>
+        <div id="log"></div>
+        <div id="box"></box>
+        <div id="box2"></box>
+        <style id="teststyles">
+            #box2 { width: 15px; }
+        </style>
+        <script>
+
+            test(function(){
+                var elem = document.getElementById('box');
+
+                elem.style.width = '10px';
+                assert_equals(elem.style.width, '10px', 'setting via style property');
+                elem.style.width = '';
+
+                elem.style.cssText = 'width: 10px';
+                assert_equals(elem.style.width, '10px', 'setting via cssText');
+                elem.style.width = '';
+
+            }, 'HTMLElement\'s CSSStyleDeclaration is mutable')
+
+
+            test(function(){
+                var elem = document.getElementById('box');
+                var style = getComputedStyle(elem);
+
+                assert_throws_dom('NO_MODIFICATION_ALLOWED_ERR', function(){
+                    style.width = '10px';
+                });
+
+            }, 'getComputedStyle\'s CSSStyleDeclaration is not mutable')
+
+
+            test(function(){
+
+                var style = document.getElementById('teststyles').sheet.cssRules[0].style;
+
+                assert_equals(style.width, '15px', 'width value is correct');
+
+                style.width = '25px';
+
+                assert_equals(style.width, '25px', 'width value is mutable');
+
+                var gCSstyle = getComputedStyle(document.getElementById('box2'));
+
+                assert_equals(gCSstyle.width, '25px', 'styleSheet change is live and accesible via getComputedStyle');
+
+            }, 'StyleSheet\'s CSSStyleDeclaration is mutable');
+
+
+        </script>
+
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-001.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-001.html
new file mode 100644
index 0000000000..ba4d926b15
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-001.html
@@ -0,0 +1,20 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: CSSStyleDeclaration.setPropertyValue queues a mutation record when changed</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setpropertyvalue">
+<link rel="help" href="https://drafts.csswg.org/cssom/#update-style-attribute-for">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script>
+  document.documentElement.style.top = "0px";
+
+  let test = async_test("CSSStyleDeclaration.setPropertyValue queues a mutation record when serialization is changed");
+  let m = new MutationObserver(function(r) {
+    assert_equals(r.length, 1);
+    test.done();
+  });
+
+  m.observe(document.documentElement,  { attributes: true });
+  document.documentElement.style.top = "1px";
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-002.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-002.html
new file mode 100644
index 0000000000..38ce249a31
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-002.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: CSSStyleDeclaration.setPropertyValue doesn't queue a mutation record for invalid values</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script>
+  let test = async_test("CSSStyleDeclaration.setPropertyValue doesn't queue a mutation record when setting invalid values");
+  let m = new MutationObserver(test.unreached_func("shouldn't queue a mutation record"));
+  m.observe(document.documentElement,  { attributes: true });
+
+  document.documentElement.style.width = "-100px";
+  requestAnimationFrame(() => test.done());
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-003.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-003.html
new file mode 100644
index 0000000000..7a99dfc645
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-003.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: CSSStyleDeclaration.removeProperty doesn't queue a mutation record when not actually removed, invoked from setPropertyValue</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setpropertyvalue">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script>
+  let test = async_test("CSSStyleDeclaration.removeProperty doesn't queue a mutation record when not actually removed, invoked from setPropertyValue");
+  document.documentElement.style.top = "0";
+  let m = new MutationObserver(test.unreached_func("shouldn't queue a mutation record"));
+  m.observe(document.documentElement,  { attributes: true });
+
+  document.documentElement.style.width = "";
+  requestAnimationFrame(() => test.done());
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-004.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-004.html
new file mode 100644
index 0000000000..55956dfdaa
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-004.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: CSSStyleDeclaration.removeProperty doesn't queue a mutation record when not actually removed</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setpropertyvalue">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script>
+  let test = async_test("CSSStyleDeclaration.removeProperty doesn't queue a mutation record when not actually removed");
+  document.documentElement.style.top = "0";
+  let m = new MutationObserver(test.unreached_func("shouldn't queue a mutation record"));
+  m.observe(document.documentElement,  { attributes: true });
+
+  document.documentElement.style.removeProperty("width");
+  requestAnimationFrame(() => test.done());
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-005.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-005.html
new file mode 100644
index 0000000000..26574033ce
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-mutationrecord-005.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<html style="color: inherit">
+<meta charset="utf-8">
+<title>CSSOM: CSSStyleDeclaration.setPropertyValue doesn't queue a mutation record for invalid values</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script>
+  let test = async_test("CSSStyleDeclaration.setPropertyValue doesn't queue a mutation record when setting invalid values");
+  let m = new MutationObserver(test.unreached_func("shouldn't queue a mutation record"));
+  m.observe(document.documentElement,  { attributes: true });
+
+  document.documentElement.style.width = "-100px";
+  requestAnimationFrame(() => test.done());
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-properties.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-properties.html
new file mode 100644
index 0000000000..12382cbcdc
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-properties.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<title>CSS Test: CSSStyleDeclaration properties are defined as WebIDL attributes, not using getOwnPropertyNames()</title>
+<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel="author" href="https://mozilla.org" title="Mozilla">
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-_camel_cased_attribute">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+  test(function() {
+    let declaration = document.documentElement.style;
+    assert_true(declaration instanceof CSSStyleDeclaration, "Should be a CSStyleDeclaration");
+    assert_true("color" in declaration, "Should support the color property");
+    assert_false(declaration.hasOwnProperty("color"), "shouldn't have an own property for WebIDL attributes");
+  });
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-registered-custom-properties.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-registered-custom-properties.html
new file mode 100644
index 0000000000..5aa4ad2532
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-registered-custom-properties.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<title>Computed CSSStyleDeclaration includes registered custom properties</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/1316">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  @property --non-inherited-length {
+    syntax: "<length>";
+    inherits: false;
+    initial-value: 0px;
+  }
+  @property --inherited-length {
+    syntax: "<length>";
+    inherits: true;
+    initial-value: 0px;
+  }
+  @property --universal-with-initial {
+    syntax: "*";
+    inherits: false;
+    initial-value: foo;
+  }
+  @property --universal-without-initial {
+    syntax: "*";
+    inherits: false;
+  }
+  #outer { --non-registered-outer: 1px; }
+  #inner { --non-registered-inner: 2px; }
+  #sibling { --universal-without-initial: bar; }
+</style>
+<div id=outer>
+  <div id=inner></div>
+  <div id=sibling></div>
+</div>
+<script>
+  const assert_present = (props, name) => assert_not_equals(props.indexOf(name), -1);
+  const assert_absent = (props, name) => assert_equals(props.indexOf(name), -1);
+
+  test(function() {
+    let props = Array.from(getComputedStyle(inner));
+    assert_present(props, '--non-inherited-length');
+    assert_present(props, '--inherited-length');
+    assert_present(props, '--non-registered-outer');
+    assert_present(props, '--non-registered-inner');
+    assert_present(props, '--universal-with-initial');
+    assert_absent(props, '--universal-without-initial');
+  }, 'Registered custom properties are included in CSSComputedStyleDeclaration');
+
+  test(function() {
+    let props = Array.from(getComputedStyle(sibling));
+    assert_present(props, '--non-inherited-length');
+    assert_present(props, '--inherited-length');
+    assert_present(props, '--non-registered-outer');
+    assert_present(props, '--universal-with-initial');
+    assert_present(props, '--universal-without-initial');
+    assert_absent(props, '--non-registered-inner');
+  }, 'Only relevant custom properties are included');
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-attr.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-attr.html
new file mode 100644
index 0000000000..20837052e3
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-attr.html
@@ -0,0 +1,18 @@
+<!doctype html>
+<title>CSSOM test: declaration block after setting via CSSOM</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script>
+test(function() {
+  let element = document.createElement("div");
+  element.style.setProperty("doesntexist", "0");
+  assert_false(element.hasAttribute("style"));
+}, "Setting an invalid property via the declaration setter doesn't create a declaration");
+test(function() {
+  let element = document.createElement("div");
+  element.style.setProperty("width", "-100");
+  assert_false(element.hasAttribute("style"));
+}, "Setting an invalid value via the declaration setter doesn't create a declaration");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-declarations.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-declarations.html
new file mode 100644
index 0000000000..e66466e7a1
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-declarations.html
@@ -0,0 +1,160 @@
+<!DOCTYPE html>
+<title>CSSOM test: declaration block after setting via CSSOM</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#set-a-css-declaration-value">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+  function generateCSSDeclBlock(props) {
+    let elem = document.createElement("div");
+    let cssText = props.map(({name, value, priority}) => {
+      let longhand = `${name}: ${value}`;
+      if (priority) {
+        longhand += "!" + priority;
+      }
+      return longhand + ";";
+    }).join(" ");
+    elem.setAttribute("style", cssText);
+    return elem.style;
+  }
+  function compareByName(a, b) {
+    if (a.name < b.name) return -1;
+    if (a.name > b.name) return 1;
+    return 0;
+  }
+  function checkDeclarationsAnyOrder(block, props, msg) {
+    let actual = [];
+    for (let name of block) {
+      let value = block.getPropertyValue(name);
+      let priority = block.getPropertyPriority(name);
+      actual.push({name, value, priority});
+    }
+    actual.sort(compareByName);
+    let expected = Array.from(props);
+    expected.sort(compareByName);
+    assert_object_equals(actual, expected, "Declaration block content should match " + msg);
+  }
+  function longhand(name, value, priority="") {
+    return {name, value, priority};
+  }
+  function* shorthand(name, value, priority="") {
+    for (let subprop of SUBPROPS[name]) {
+      yield longhand(subprop, value, priority);
+    }
+  }
+
+  const SUBPROPS = {
+    "margin": ["margin-top", "margin-right", "margin-bottom", "margin-left"],
+    "padding": ["padding-top", "padding-right", "padding-bottom", "padding-left"],
+  };
+
+  test(function() {
+    let expectedDecls = [
+      longhand("top", "1px"),
+      longhand("bottom", "2px"),
+      longhand("left", "3px", "important"),
+      longhand("right", "4px"),
+    ];
+    let block = generateCSSDeclBlock(expectedDecls);
+    checkDeclarationsAnyOrder(block, expectedDecls, "in initial block");
+
+    block.setProperty("top", "5px", "important");
+    expectedDecls[0] = longhand("top", "5px", "important");
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting existing property");
+
+    block.setProperty("bottom", "2px");
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting existing property with identical value");
+
+    block.setProperty("left", "3px");
+    expectedDecls[2].priority = "";
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting existing property with different priority");
+
+    block.setProperty("float", "none");
+    expectedDecls.push(longhand("float", "none"));
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting non-existing property");
+  }, "setProperty with longhand should update only the declaration being set");
+
+  test(function() {
+    let expectedDecls = [
+      longhand("top", "1px"),
+      longhand("bottom", "2px"),
+      longhand("left", "3px", "important"),
+      longhand("right", "4px"),
+    ];
+    let block = generateCSSDeclBlock(expectedDecls);
+    checkDeclarationsAnyOrder(block, expectedDecls, "in initial block");
+
+    block.top = "5px";
+    expectedDecls[0] = longhand("top", "5px");
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting existing property");
+
+    block.bottom = "2px";
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting existing property with identical value");
+
+    block.left = "3px";
+    expectedDecls[2].priority = "";
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting existing property with different priority");
+
+    block.float = "none";
+    expectedDecls.push(longhand("float", "none"));
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting non-existing property");
+  }, "property setter should update only the declaration being set");
+
+  test(function() {
+    let expectedDecls = [
+      ...shorthand("margin", "1px"),
+      longhand("top", "2px"),
+      ...shorthand("padding", "3px", "important"),
+    ];
+    let block = generateCSSDeclBlock(expectedDecls);
+    checkDeclarationsAnyOrder(block, expectedDecls, "in initial block");
+
+    block.setProperty("margin", "4px");
+    for (let i = 0; i < 4; i++) {
+      expectedDecls[i].value = "4px";
+    }
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting an existing shorthand");
+
+    block.setProperty("margin", "4px");
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting an existing shorthand with identical value");
+
+    block.setProperty("padding", "3px");
+    for (let i = 5; i < 9; i++) {
+      expectedDecls[i].priority = "";
+    }
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting an existing shorthand with different priority");
+
+    block.setProperty("margin-bottom", "5px", "important");
+    expectedDecls[2] = longhand("margin-bottom", "5px", "important");
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting a longhand in an existing shorthand");
+  }, "setProperty with shorthand should update only the declarations being set");
+
+  test(function() {
+    let expectedDecls = [
+      ...shorthand("margin", "1px"),
+      longhand("top", "2px"),
+      ...shorthand("padding", "3px", "important"),
+    ];
+    let block = generateCSSDeclBlock(expectedDecls);
+    checkDeclarationsAnyOrder(block, expectedDecls, "in initial block");
+
+    block.margin = "4px";
+    for (let i = 0; i < 4; i++) {
+      expectedDecls[i].value = "4px";
+    }
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting an existing shorthand");
+
+    block.margin = "4px";
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting an existing shorthand with identical value");
+
+    block.padding = "3px";
+    for (let i = 5; i < 9; i++) {
+      expectedDecls[i].priority = "";
+    }
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting an existing shorthand with different priority");
+
+    block.marginBottom = "5px";
+    expectedDecls[2] = longhand("margin-bottom", "5px");
+    checkDeclarationsAnyOrder(block, expectedDecls, "after setting a longhand in an existing shorthand");
+  }, "longhand property setter should update only the decoarations being set");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-form-controls.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-form-controls.html
new file mode 100644
index 0000000000..ae556ed1cb
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-form-controls.html
@@ -0,0 +1,103 @@
+<!doctype html>
+<title>CSSOM test: no side effects from setting "height"</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty">
+<link rel="help" href="https://bugs.webkit.org/show_bug.cgi?id=107380">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<p>
+Historically, the Apple Safari web browser has added an "intrinsic margin" to
+form controls which do not specify a `height`. It removed this margin following
+modification of the `height`. <a
+href="https://bugs.webkit.org/show_bug.cgi?id=107380">This non-standard
+behavior was identified as a source of confusion for web developers.</a>
+</p>
+
+<script>
+function makeElement(tagName) {
+  var element = document.createElement(tagName);
+  document.body.appendChild(element);
+  return element;
+}
+function makeInputElement(type) {
+  var element = makeElement('input');
+  element.setAttribute('type', type);
+  return element;
+}
+function measure(element) {
+  var computed = getComputedStyle(element);
+  return [computed.marginTop, computed.marginBottom];
+}
+
+test(function() {
+  var element = makeInputElement('text');
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'text input element');
+
+test(function() {
+  var element = makeInputElement('button');
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'button input element');
+
+test(function() {
+  var element = makeInputElement('submit');
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'submit input element');
+
+test(function() {
+  var element = makeInputElement('radio');
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'radio input element');
+
+test(function() {
+  var element = makeInputElement('checkbox');
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'checkbox input element');
+
+test(function() {
+  var element = makeElement('textarea');
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'textarea element');
+
+test(function() {
+  var element = makeElement('select');
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'select element');
+
+test(function() {
+  var element = makeElement('button')
+  var initial = measure(element);
+
+  element.style.setProperty('height', '12px');
+
+  assert_array_equals(measure(element), initial);
+}, 'button element');
+</script>
diff --git a/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-logical.html b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-logical.html
new file mode 100644
index 0000000000..13d68e9a70
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/cssstyledeclaration-setter-logical.html
@@ -0,0 +1,73 @@
+<!DOCTYPE html>
+<title>CSSOM test: declaration block after setting via CSSOM</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#set-a-css-declaration-value">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<div id="test"></div>
+<script>
+  test(function() {
+    const PHYSICAL_PROPS = ["padding-top", "padding-right",
+                            "padding-bottom", "padding-left"];
+    const LOGICAL_PROPS = ["padding-block-start", "padding-block-end",
+                           "padding-inline-start", "padding-inline-end"];
+    let elem = document.getElementById("test");
+    let decls = new Map;
+
+    {
+      let css = []
+      let i = 0;
+      for (let name of [...PHYSICAL_PROPS, ...LOGICAL_PROPS]) {
+        let value = `${i++}px`;
+        css.push(`${name}: ${value};`);
+        decls.set(name, value);
+      }
+      elem.setAttribute("style", css.join(" "));
+    }
+
+    let style = elem.style;
+    function indexOfProperty(prop) {
+      return Array.prototype.indexOf.apply(style, [prop]);
+    }
+    function assertPropAfterProps(prop, props, msg) {
+      let index = indexOfProperty(prop);
+      for (let p of props) {
+        assert_less_than(indexOfProperty(p), index, `${prop} should be after ${p} ${msg}`);
+      }
+    }
+    // We are not changing any value in this test, just order.
+    function assertValueUnchanged() {
+      for (let [name, value] of decls.entries()) {
+        assert_equals(style.getPropertyValue(name), value,
+                      `value of ${name} shouldn't be changed`);
+      }
+    }
+
+    assertValueUnchanged();
+    // Check that logical properties are all after physical properties
+    // at the beginning.
+    for (let p of LOGICAL_PROPS) {
+      assertPropAfterProps(p, PHYSICAL_PROPS, "initially");
+    }
+
+    // Try setting a longhand.
+    style.setProperty("padding-top", "0px");
+    assertValueUnchanged();
+    // Check that padding-top is after logical properties, but other
+    // physical properties are still before logical properties.
+    assertPropAfterProps("padding-top", LOGICAL_PROPS, "after setting padding-top");
+    for (let p of LOGICAL_PROPS) {
+      assertPropAfterProps(p, PHYSICAL_PROPS.filter(prop => prop != "padding-top"),
+                           "after setting padding-top");
+    }
+
+    // Try setting a shorthand.
+    style.setProperty("padding", "0px 1px 2px 3px");
+    assertValueUnchanged();
+    // Check that all physical properties are now after logical properties.
+    for (let p of PHYSICAL_PROPS) {
+      assertPropAfterProps(p, LOGICAL_PROPS, "after setting padding shorthand");
+    }
+  }, "newly set declaration should be after all declarations " +
+  "in the same logical property group but have different logical kind");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/declaration-block-all-crash.html b/testing/web-platform/tests/css/cssom/declaration-block-all-crash.html
new file mode 100644
index 0000000000..2d781e5dcf
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/declaration-block-all-crash.html
@@ -0,0 +1,10 @@
+<!doctype html>
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1711189">
+<div></div>
+<script>
+  let div = document.querySelector("div");
+  let cs = getComputedStyle(div);
+  for (let i = 0; i < cs.length; ++i)
+    div.style.setProperty(cs[i], cs.getPropertyValue(cs[i]));
+  div.style.cssText
+</script>
diff --git a/testing/web-platform/tests/css/cssom/escape.html b/testing/web-platform/tests/css/cssom/escape.html
new file mode 100644
index 0000000000..895da59c49
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/escape.html
@@ -0,0 +1,102 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>CSS#escape</title>
+<link rel=help href=https://drafts.csswg.org/cssom-1/#the-css.escape()-method>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<div id=log></div>
+<script>
+test(function() {
+  assert_equals(CSS.escape.length, 1);
+  assert_throws_js(TypeError, function() { CSS.escape(); });
+}, "Number of arguments");
+
+test(function() {
+  assert_equals(CSS.escape(true), 'true');
+  assert_equals(CSS.escape(false), 'false');
+  assert_equals(CSS.escape(null), 'null');
+  assert_equals(CSS.escape(''), '');
+}, "String conversion");
+
+test(function() {
+  assert_equals(CSS.escape('\0'), '\uFFFD');
+  assert_equals(CSS.escape('a\0'), 'a\uFFFD');
+  assert_equals(CSS.escape('\0b'), '\uFFFDb');
+  assert_equals(CSS.escape('a\0b'), 'a\uFFFDb');
+}, "Null bytes");
+
+test(function() {
+  assert_equals(CSS.escape('\uFFFD'), '\uFFFD');
+  assert_equals(CSS.escape('a\uFFFD'), 'a\uFFFD');
+  assert_equals(CSS.escape('\uFFFDb'), '\uFFFDb');
+  assert_equals(CSS.escape('a\uFFFDb'), 'a\uFFFDb');
+}, "Replacement character");
+
+test(function() {
+  assert_equals(CSS.escape('0a'), '\\30 a');
+  assert_equals(CSS.escape('1a'), '\\31 a');
+  assert_equals(CSS.escape('2a'), '\\32 a');
+  assert_equals(CSS.escape('3a'), '\\33 a');
+  assert_equals(CSS.escape('4a'), '\\34 a');
+  assert_equals(CSS.escape('5a'), '\\35 a');
+  assert_equals(CSS.escape('6a'), '\\36 a');
+  assert_equals(CSS.escape('7a'), '\\37 a');
+  assert_equals(CSS.escape('8a'), '\\38 a');
+  assert_equals(CSS.escape('9a'), '\\39 a');
+}, "Number prefix");
+
+test(function() {
+  assert_equals(CSS.escape('a0b'), 'a0b');
+  assert_equals(CSS.escape('a1b'), 'a1b');
+  assert_equals(CSS.escape('a2b'), 'a2b');
+  assert_equals(CSS.escape('a3b'), 'a3b');
+  assert_equals(CSS.escape('a4b'), 'a4b');
+  assert_equals(CSS.escape('a5b'), 'a5b');
+  assert_equals(CSS.escape('a6b'), 'a6b');
+  assert_equals(CSS.escape('a7b'), 'a7b');
+  assert_equals(CSS.escape('a8b'), 'a8b');
+  assert_equals(CSS.escape('a9b'), 'a9b');
+}, "Letter number prefix");
+
+test(function() {
+  assert_equals(CSS.escape('-0a'), '-\\30 a');
+  assert_equals(CSS.escape('-1a'), '-\\31 a');
+  assert_equals(CSS.escape('-2a'), '-\\32 a');
+  assert_equals(CSS.escape('-3a'), '-\\33 a');
+  assert_equals(CSS.escape('-4a'), '-\\34 a');
+  assert_equals(CSS.escape('-5a'), '-\\35 a');
+  assert_equals(CSS.escape('-6a'), '-\\36 a');
+  assert_equals(CSS.escape('-7a'), '-\\37 a');
+  assert_equals(CSS.escape('-8a'), '-\\38 a');
+  assert_equals(CSS.escape('-9a'), '-\\39 a');
+}, "Dash number prefix");
+
+test(function() {
+  assert_equals(CSS.escape('--a'), '--a');
+}, "Double dash prefix");
+
+test(function() {
+  assert_equals(CSS.escape('\x01\x02\x1E\x1F'), '\\1 \\2 \\1e \\1f ');
+
+  assert_equals(CSS.escape('\x80\x2D\x5F\xA9'), '\x80\x2D\x5F\xA9');
+  assert_equals(CSS.escape('\x7F\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8A\x8B\x8C\x8D\x8E\x8F\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9A\x9B\x9C\x9D\x9E\x9F'), '\\7f \x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8A\x8B\x8C\x8D\x8E\x8F\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9A\x9B\x9C\x9D\x9E\x9F');
+  assert_equals(CSS.escape('\xA0\xA1\xA2'), '\xA0\xA1\xA2');
+  assert_equals(CSS.escape('a0123456789b'), 'a0123456789b');
+  assert_equals(CSS.escape('abcdefghijklmnopqrstuvwxyz'), 'abcdefghijklmnopqrstuvwxyz');
+  assert_equals(CSS.escape('ABCDEFGHIJKLMNOPQRSTUVWXYZ'), 'ABCDEFGHIJKLMNOPQRSTUVWXYZ');
+
+  assert_equals(CSS.escape("hello\\world"), "hello\\\\world", "Backslashes get backslash-escaped");
+  assert_equals(CSS.escape("hello\u{1234}world"), "hello\u{1234}world", "Code points greater than U+0080 are preserved");
+  assert_equals(CSS.escape("-"), "\\-", "CSS.escape: Single dash escaped");
+
+  assert_equals(CSS.escape('\x20\x21\x78\x79'), '\\ \\!xy');
+}, "Various tests");
+
+test(function() {
+  // astral symbol (U+1D306 TETRAGRAM FOR CENTRE)
+  assert_equals(CSS.escape('\uD834\uDF06'), '\uD834\uDF06');
+  // lone surrogates
+  assert_equals(CSS.escape('\uDF06'), '\uDF06');
+  assert_equals(CSS.escape('\uD834'), '\uD834');
+}, "Surrogates");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/flex-serialization.html b/testing/web-platform/tests/css/cssom/flex-serialization.html
new file mode 100644
index 0000000000..350877c3ce
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/flex-serialization.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - Flex serialization</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-value">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style>
+        div { flex: initial; }
+        div { flex: 0; }
+        div { flex: initial; flex-basis: initial; flex-shrink: initial; }
+        div { flex: initial; flex-shrink: 0; }
+        div { flex: initial; flex-basis: 0; flex-shrink: 2;}
+    </style>
+
+    <script>
+    var styleSheet = document.styleSheets[0]
+    test(function () {
+        assert_equals(styleSheet.cssRules[0].style.cssText, "flex: initial;")
+    }, "Single value flex with CSS-wide keyword should serialize correctly.")
+    test(function () {
+        assert_in_array(styleSheet.cssRules[1].style.cssText, ["flex: 0px;", "flex: 0 1 0px;"])
+    }, "Single value flex with non-CSS-wide value should serialize correctly.")
+    test(function () {
+        assert_equals(styleSheet.cssRules[2].style.cssText, "flex: initial;")
+    }, "Multiple values flex with CSS-wide keyword should serialize correctly.")
+    test(function () {
+        assert_equals(styleSheet.cssRules[3].style.cssText, "flex-grow: initial; flex-basis: initial; flex-shrink: 0;")
+    }, "Multiple values flex with CSS-wide keywords and non-CSS-wide value should serialize correctly.")
+    test(function () {
+        assert_equals(styleSheet.cssRules[4].style.cssText, "flex-grow: initial; flex-basis: 0px; flex-shrink: 2;")
+    }, "Multiple values flex with CSS-wide and two non-CSS-wide-keyword values should serialize correctly.")
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/font-family-serialization-001.html b/testing/web-platform/tests/css/cssom/font-family-serialization-001.html
new file mode 100644
index 0000000000..436ce7f5c1
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/font-family-serialization-001.html
@@ -0,0 +1,44 @@
+<!DOCTYPE HTML>
+<html>
+<meta charset="utf-8">
+<title>Serialization of font-family</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-declaration-block">
+<link rel="help" href="https://drafts.csswg.org/css-fonts-4/#font-family-prop">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/css-fonts/support/font-family-keywords.js"></script>
+<div id="target"></div>
+<script>
+  function SetFontFamilyAndSerialize(fontFamilyValue) {
+    var target = document.getElementById('target');
+    target.setAttribute("style", `font-family: ${fontFamilyValue}`);
+    return window.getComputedStyle(target).getPropertyValue('font-family');
+  }
+  test(function() {
+    kGenericFontFamilyKeywords.forEach(keyword => {
+      assert_equals(SetFontFamilyAndSerialize(keyword), keyword);
+    });
+  }, "Serialization of <generic-family>");
+
+  test(function() {
+    kGenericFontFamilyKeywords.forEach(keyword => {
+      var quoted_keyword = `"${keyword}"`;
+      assert_equals(SetFontFamilyAndSerialize(quoted_keyword), quoted_keyword);
+    });
+  }, "Serialization of quoted \"<generic-family>\"");
+
+  test(function() {
+    kGenericFontFamilyKeywords.forEach(keyword => {
+      var prefixed_keyword = `-webkit-${keyword}`;
+      assert_equals(SetFontFamilyAndSerialize(prefixed_keyword), prefixed_keyword);
+    });
+  }, "Serialization of prefixed -webkit-<generic-family>");
+
+  test(function() {
+    kNonGenericFontFamilyKeywords.forEach(keyword => {
+      assert_equals(SetFontFamilyAndSerialize(keyword), keyword);
+    });
+  }, `Serialization of ${kNonGenericFontFamilyKeywords}`);
+
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/font-shorthand-serialization.html b/testing/web-platform/tests/css/cssom/font-shorthand-serialization.html
new file mode 100644
index 0000000000..29082f83ea
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/font-shorthand-serialization.html
@@ -0,0 +1,17 @@
+<!doctype html>
+<html>
+<meta charset="utf-8">
+<title>Serialization of font shorthand</title>
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#serialize-a-css-declaration-block">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="stylesheet" type="text/css" href="/fonts/ahem.css" />
+<div id="target" style="font: 10px/1 Ahem;"></div>
+<script>
+    test(function() {
+        var target = document.getElementById('target');
+        assert_equals(target.style.cssText, 'font: 10px / 1 Ahem;');
+        assert_equals(target.style.font, '10px / 1 Ahem');
+    }, "The font shorthand should be serialized just like any other shorthand.");
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/font-variant-shorthand-serialization.html b/testing/web-platform/tests/css/cssom/font-variant-shorthand-serialization.html
new file mode 100644
index 0000000000..0e4651cdc0
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/font-variant-shorthand-serialization.html
@@ -0,0 +1,131 @@
+<!doctype html>
+<html>
+<meta charset="utf-8">
+<title>Serialization of font-variant shorthand</title>
+<link rel="author" title="Tim Nguyen" href="https://github.com/nt1m">
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#serialize-a-css-declaration-block">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="stylesheet" type="text/css" href="/fonts/ahem.css" />
+<div id="target"></div>
+<script>
+    const cssWideKeywords = ["initial", "inherit", "unset", "revert", "revert-layer"];
+    function test_serialization_set(expected) {
+        for (const [property, value] of Object.entries(expected)) {
+            if (!CSS.supports(`${property}: initial`))
+                continue;
+            assert_equals(target.style[property], value, `${property} was set`);
+        }
+    }
+    function setWithValue(value) {
+        return {
+            "font-variant-ligatures": value,
+            "font-variant-caps": value,
+            "font-variant-alternates": value,
+            "font-variant-numeric": value,
+            "font-variant-east-asian": value,
+            "font-variant-position": value,
+            "font-variant-emoji": value,
+            "font-variant": value
+        };
+    }
+    const emptySet = setWithValue("");
+    const normalSet = setWithValue("normal");
+    const nonDefaultValues = {
+        "font-variant-ligatures": "common-ligatures discretionary-ligatures",
+        "font-variant-caps": "small-caps",
+        "font-variant-alternates": "historical-forms",
+        "font-variant-numeric": "oldstyle-nums stacked-fractions",
+        "font-variant-east-asian": "ruby",
+        "font-variant-position": "sub",
+        "font-variant-emoji": "emoji",
+    };
+    test(function(t) {
+        target.style.fontVariant = "normal";
+        t.add_cleanup(() => target.removeAttribute("style"));
+
+        test_serialization_set(normalSet);
+    }, "font-variant: normal serialization");
+
+    test(function(t) {
+        target.style.fontVariant = "normal";
+        target.style.fontVariantLigatures = "none";
+        t.add_cleanup(() => target.removeAttribute("style"));
+
+        const expected = Object.assign({}, normalSet);
+        expected["font-variant-ligatures"] = "none";
+        expected["font-variant"] = "none";
+
+        test_serialization_set(expected);
+    }, "font-variant: none serialization");
+
+    test(function(t) {
+        t.add_cleanup(() => target.removeAttribute("style"));
+        for (const [property, value] of Object.entries(nonDefaultValues)) {
+            if (property == "font-variant-ligatures" || !CSS.supports(`${property}: initial`))
+                continue;
+            target.style.fontVariant = "normal";
+            target.style.fontVariantLigatures = "none";
+            target.style[property] = value;
+
+            const expected = Object.assign({}, normalSet);
+            expected["font-variant-ligatures"] = "none";
+            expected["font-variant"] = "";
+            expected[property] = value;
+
+            test_serialization_set(expected);
+            target.removeAttribute("style");
+        }
+    }, "font-variant-ligatures: none serialization with non-default value for another longhand");
+
+    test(function(t) {
+        t.add_cleanup(() => target.removeAttribute("style"));
+
+        for (const [property, value] of Object.entries(nonDefaultValues)) {
+            if (!CSS.supports(`${property}: initial`))
+                continue;
+            target.style.fontVariant = "normal";
+            target.style[property] = value;
+
+            const expected = Object.assign({}, normalSet);
+            expected[property] = value;
+            expected["font-variant"] = value;
+            test_serialization_set(expected);
+
+            target.removeAttribute("style");
+        }
+    }, "font-variant: normal with non-default longhands");
+
+    test(function(t) {
+        t.add_cleanup(() => target.removeAttribute("style"));
+        for (const keyword of cssWideKeywords) {
+            target.style.fontVariant = "normal";
+            target.style.fontVariantLigatures = keyword;
+
+            const expected = Object.assign({}, normalSet);
+            expected["font-variant-ligatures"] = keyword;
+            expected["font-variant"] = "";
+            test_serialization_set(expected);
+            target.removeAttribute("style");
+        }
+    }, "CSS-wide keyword in one longhand");
+
+    test(function(t) {
+        t.add_cleanup(() => target.removeAttribute("style"));
+
+        for (const keyword of cssWideKeywords) {
+            target.style.fontVariant = keyword;
+            test_serialization_set(setWithValue(keyword));
+            target.removeAttribute("style");
+        }
+    }, "CSS-wide keyword in shorthand");
+
+    test(function(t) {
+        target.style.fontVariant = "normal";
+        target.style.font = "menu";
+        t.add_cleanup(() => target.removeAttribute("style"));
+
+        test_serialization_set(emptySet);
+    }, "font: menu serialization");
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-animations-replaced-into-ib-split.html b/testing/web-platform/tests/css/cssom/getComputedStyle-animations-replaced-into-ib-split.html
new file mode 100644
index 0000000000..47198803a0
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-animations-replaced-into-ib-split.html
@@ -0,0 +1,34 @@
+<!doctype html>
+<title>getComputedStyle() returns the right style for animating nodes that have been just inserted into the document, and that have an ancestor whose layout tree was recreated (like an IB-split)</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1585882">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style>
+  @keyframes my-animation {
+    from { color: green; }
+    to { color: green; }
+  }
+  div {
+    color: red;
+    animation: my-animation 1s infinite linear paused;
+  }
+</style>
+<span>
+  <div></div>
+</span>
+<script>
+test(() => {
+  let oldDiv = document.querySelector("div");
+  window.unused = oldDiv.getBoundingClientRect(); // update layout
+
+  assert_equals(getComputedStyle(oldDiv).color, "rgb(0, 128, 0)", "Should take color from the animation");
+
+  let newDiv = document.createElement("div");
+  oldDiv.replaceWith(newDiv);
+
+  assert_equals(getComputedStyle(newDiv).color, "rgb(0, 128, 0)", "Should take color from the animation (just inserted into the document)");
+}, "getComputedStyle() should return animation styles for nodes just inserted into the document, even if they're in an IB-split");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-detached-subtree.html b/testing/web-platform/tests/css/cssom/getComputedStyle-detached-subtree.html
new file mode 100644
index 0000000000..211d512b28
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-detached-subtree.html
@@ -0,0 +1,49 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: getComputedStyle returns no style for elements not in the tree</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<div id="host">
+  <div id="non-slotted">
+    <div id="non-slotted-descendant"></div>
+  </div>
+</div>
+<iframe srcdoc="<html></html>" style="display: none"></iframe>
+<script>
+function testNoComputedStyle(element, description, global) {
+  test(function() {
+    assert_true(!!element);
+    let style = (global ? global : window).getComputedStyle(element);
+    assert_true(!!style);
+    // Note that we avoid assert_equals below to get a stable failure message.
+    assert_true(style.length == 0);
+    assert_equals(style.color, "");
+  }, `getComputedStyle returns no style for ${description}`);
+}
+
+let detached = document.createElement('div');
+testNoComputedStyle(detached, "detached element");
+
+testNoComputedStyle(document.querySelector('iframe').contentDocument.documentElement,
+                    "element in non-rendered iframe (display: none)");
+
+testNoComputedStyle(document.querySelector('iframe').contentDocument.documentElement,
+                    "element in non-rendered iframe (display: none) from iframe's window",
+                    document.querySelector('iframe').contentWindow);
+
+host.attachShadow({ mode: "open" });
+testNoComputedStyle(document.getElementById('non-slotted'),
+                    "element outside the flat tree");
+
+testNoComputedStyle(document.getElementById('non-slotted-descendant'),
+                    "descendant outside the flat tree");
+
+let shadowRoot = detached.attachShadow({ mode: "open" });
+shadowRoot.innerHTML = `
+  <div id="detached-shadow-tree-descendant"></div>
+`;
+testNoComputedStyle(shadowRoot.getElementById('detached-shadow-tree-descendant'),
+                    "shadow tree outside of flattened tree");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-001.html b/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-001.html
new file mode 100644
index 0000000000..5a356f15a4
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-001.html
@@ -0,0 +1,39 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: getComputedStyle gets invalidated for display: none elements (inheritance)</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://bugs.webkit.org/show_bug.cgi?id=186882">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style>
+#undisplayed, #host {
+  display: none;
+  color: red;
+}
+</style>
+<div id="undisplayed"><div id="child"></div></div>
+<div id="host"></div>
+<script>
+  test(function() {
+    let undisplayed_style = getComputedStyle(undisplayed);
+    let undisplayed_child_style = getComputedStyle(child);
+    assert_equals(undisplayed_style.color, "rgb(255, 0, 0)");
+    assert_equals(undisplayed_child_style.color, "rgb(255, 0, 0)");
+    undisplayed.style.color = "green";
+    assert_equals(undisplayed_style.color, "rgb(0, 128, 0)");
+    assert_equals(undisplayed_child_style.color, "rgb(0, 128, 0)");
+  }, "getComputedStyle gets invalidated in display: none subtrees due to inherited changes to an ancestor");
+  test(function() {
+    host.attachShadow({ mode: 'open' }).innerHTML = `
+      <div></div>
+    `;
+    let host_style = getComputedStyle(host);
+    let shadow_style = getComputedStyle(host.shadowRoot.firstElementChild);
+    assert_equals(host_style.color, "rgb(255, 0, 0)");
+    assert_equals(shadow_style.color, "rgb(255, 0, 0)");
+    host.style.color = "green";
+    assert_equals(host_style.color, "rgb(0, 128, 0)");
+    assert_equals(shadow_style.color, "rgb(0, 128, 0)");
+  }, "getComputedStyle gets invalidated in display: none subtrees due to inherited changes to an ancestor shadow host");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-002.html b/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-002.html
new file mode 100644
index 0000000000..12fb7fc3f8
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-002.html
@@ -0,0 +1,34 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: getComputedStyle gets invalidated for display: none elements (rules)</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://bugs.webkit.org/show_bug.cgi?id=186882">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style>
+#undisplayed {
+  display: none;
+  color: red;
+}
+.sibling + #undisplayed {
+  color: green;
+}
+
+.sibling + #undisplayed > div {
+  color: yellow;
+}
+</style>
+<div id="helper"></div>
+<div id="undisplayed"><div id="child"></div></div>
+<script>
+  test(function() {
+    let undisplayed_style = getComputedStyle(undisplayed);
+    let undisplayed_child_style = getComputedStyle(child);
+    assert_equals(undisplayed_style.color, "rgb(255, 0, 0)");
+    assert_equals(undisplayed_child_style.color, "rgb(255, 0, 0)");
+    helper.classList.add("sibling");
+    assert_equals(undisplayed_style.color, "rgb(0, 128, 0)");
+    assert_equals(undisplayed_child_style.color, "rgb(255, 255, 0)");
+  }, "getComputedStyle gets invalidated in display: none subtrees due to rule matching changes");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-003.html b/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-003.html
new file mode 100644
index 0000000000..7ea4505588
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-display-none-003.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<title>style is invalidated properly as a result of attribute changes in display: none subtrees</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style>
+div {
+  color: red
+}
+.foo {
+  color: lime;
+}
+</style>
+<div style="display: none">
+  <div id="undisplayed"></div>
+</div>
+<script>
+  test(function() {
+    let undisplayed_style = getComputedStyle(undisplayed);
+    assert_equals(undisplayed_style.color, "rgb(255, 0, 0)");
+    undisplayed.classList.add("foo");
+    assert_equals(undisplayed_style.color, "rgb(0, 255, 0)");
+  }, "getComputedStyle gets invalidated in display: none subtrees due to attribute mutations");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-dynamic-subdoc.html b/testing/web-platform/tests/css/cssom/getComputedStyle-dynamic-subdoc.html
new file mode 100644
index 0000000000..aa49dc33ef
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-dynamic-subdoc.html
@@ -0,0 +1,33 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: getComputedStyle cross-doc properly reflects media query changes</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<iframe id="frm" style="width: 100px; height: 100px"></iframe>
+<script>
+test(function() {
+  let frm = document.getElementById('frm');
+  let frmDoc = frm.contentWindow.document;
+  frmDoc.open();
+  frmDoc.write('<style>body { color: red } @media all and (min-width: 101px) { body { color: green } }</style><body>Should be green</body>');
+  frmDoc.close();
+
+  document.body.offsetTop;
+
+  assert_equals(
+    getComputedStyle(frmDoc.body).color,
+    "rgb(255, 0, 0)",
+    "Initial color should be red"
+  );
+
+  frm.style.width = "200px";
+
+  assert_equals(
+    getComputedStyle(frmDoc.body).color,
+    "rgb(0, 128, 0)",
+    "style should have been updated to account for media query changes"
+  );
+}, "getComputedStyle cross-doc properly reflects media query changes");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-getter-v-properties.tentative.html b/testing/web-platform/tests/css/cssom/getComputedStyle-getter-v-properties.tentative.html
new file mode 100644
index 0000000000..e61241e570
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-getter-v-properties.tentative.html
@@ -0,0 +1,34 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSStyleDeclaration index getter v. attributes</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/2529">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="testElement"></div>
+<script>
+
+/* per spec, the indexed getter gives all supported longhand properties, whereas
+   attributes are created for all supported properties; this implies the indexed
+   getter gives a subset of the attributes */
+
+const decl = window.getComputedStyle(document.getElementById("testElement"));
+const declItems = Array.from(decl).sort();
+
+const shorthands = ['border-top', 'border-right', 'border-bottom', 'border-left', 'border', 'font'];
+const non_shorthands = ['margin-top', 'font-size', 'max-width', 'width'];
+
+for (const prop of shorthands) {
+  test(() => {
+    assert_true(prop in decl, "getComputedStyle attribute");
+    assert_false(declItems.includes(prop), "getComputedStyle indexed getter");
+  }, prop);
+}
+
+for (const prop of non_shorthands) {
+  test(() => {
+    assert_true(prop in decl, "getComputedStyle attribute");
+    assert_true(declItems.includes(prop), "getComputedStyle indexed getter");
+  }, prop);
+}
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-insets-absolute.html b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-absolute.html
new file mode 100644
index 0000000000..fae0a84a7b
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-absolute.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the inset properties for absolute positioning</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="help" href="https://drafts.csswg.org/css-position/#pos-sch">
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script type="module">
+import {runTests, containerForAbspos} from "./support/getComputedStyle-insets.js";
+runTests({
+  style: "position: absolute",
+  containingBlockElement: containerForAbspos,
+  containingBlockArea: "padding",
+  preservesPercentages: false,
+  preservesAuto: false,
+  canStretchAutoSize: true,
+  staticPositionY: 1 + 2 + 4 + 8,
+  staticPositionX: 2 + 4 + 8 + 16,
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-insets-fixed.html b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-fixed.html
new file mode 100644
index 0000000000..ff80d2fc50
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-fixed.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the inset properties for fixed positioning</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="help" href="https://drafts.csswg.org/css-position/#pos-sch">
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script type="module">
+import {runTests, containerForFixed} from "./support/getComputedStyle-insets.js";
+runTests({
+  style: "position: fixed",
+  containingBlockElement: containerForFixed,
+  containingBlockArea: "padding",
+  preservesPercentages: false,
+  preservesAuto: false,
+  canStretchAutoSize: true,
+  staticPositionY: 1 + 2 + 4 + 8 + 16 + 32 + 64,
+  staticPositionX: 2 + 4 + 8 + 16 + 32 + 64 + 128,
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-insets-nobox.html b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-nobox.html
new file mode 100644
index 0000000000..236abf17f9
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-nobox.html
@@ -0,0 +1,18 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the inset properties when the element generates no box</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="help" href="https://drafts.csswg.org/css-position/#pos-sch">
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script type="module">
+import {runTests} from "./support/getComputedStyle-insets.js";
+runTests({
+  style: "display: none",
+  containingBlockElement: null,
+  preservesPercentages: true,
+  preservesAuto: true,
+  canStretchAutoSize: false,
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-insets-relative.html b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-relative.html
new file mode 100644
index 0000000000..1abfc62c01
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-relative.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the inset properties for relative positioning</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="help" href="https://drafts.csswg.org/css-position/#pos-sch">
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script type="module">
+import {runTests, containerForInflow} from "./support/getComputedStyle-insets.js";
+runTests({
+  style: "position: relative",
+  containingBlockElement: containerForInflow,
+  containingBlockArea: "content",
+  preservesPercentages: false,
+  preservesAuto: false,
+  canStretchAutoSize: false,
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-insets-static.html b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-static.html
new file mode 100644
index 0000000000..aa7dbee956
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-static.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the inset properties for static positioning</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="help" href="https://drafts.csswg.org/css-position/#pos-sch">
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script type="module">
+import {runTests, containerForInflow} from "./support/getComputedStyle-insets.js";
+runTests({
+  style: "position: static",
+  containingBlockElement: containerForInflow,
+  containingBlockArea: "content",
+  preservesPercentages: true,
+  preservesAuto: true,
+  canStretchAutoSize: false,
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-insets-sticky-container-for-abspos.html b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-sticky-container-for-abspos.html
new file mode 100644
index 0000000000..7f5ec30648
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-sticky-container-for-abspos.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the inset properties for sticky positioning</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="help" href="https://drafts.csswg.org/css-position/#sticky-pos">
+<link rel="author" title="Joonghun Park" href="mailto:pjh0718@gmail.com">
+<style>
+    #container-for-abspos {
+        height: 200px;
+        width: 400px;
+        overflow: hidden;
+    }
+</style>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script type="module">
+import {runTests, containerForAbspos} from "./support/getComputedStyle-insets.js";
+runTests({
+  style: "position: sticky;",
+  containingBlockElement: containerForAbspos,
+  containingBlockArea: "content",
+  preservesPercentages: false,
+  preservesAuto: true,
+  canStretchAutoSize: false,
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-insets-sticky.html b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-sticky.html
new file mode 100644
index 0000000000..6b23fabcb1
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-insets-sticky.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the inset properties for sticky positioning</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="help" href="https://drafts.csswg.org/css-position/#pos-sch">
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<style>
+    #container-for-inflow { overflow: hidden; }
+</style>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script type="module">
+import {runTests, containerForInflow} from "./support/getComputedStyle-insets.js";
+runTests({
+  style: "position: sticky;",
+  containingBlockElement: containerForInflow,
+  containingBlockArea: "content",
+  preservesPercentages: false,
+  preservesAuto: true,
+  canStretchAutoSize: false,
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-layout-dependent-removed-ib-sibling.html b/testing/web-platform/tests/css/cssom/getComputedStyle-layout-dependent-removed-ib-sibling.html
new file mode 100644
index 0000000000..83482c5f5c
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-layout-dependent-removed-ib-sibling.html
@@ -0,0 +1,30 @@
+<!doctype html>
+<title>getComputedStyle() returns the right style for layout-dependent properties for nodes that have had an IB sibling removed</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1585882">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style>
+  div {
+    width: 100%;
+  }
+</style>
+<span>
+  <div></div>
+  <div></div>
+</span>
+<script>
+test(() => {
+  let first = document.querySelector("div");
+  let second = document.querySelector("div + div");
+
+  let oldWidth = getComputedStyle(second).width;
+  assert_true(oldWidth.indexOf("px") !== -1, "Should return the used value for width");
+
+  first.remove();
+
+  assert_equals(getComputedStyle(second).width, oldWidth, "Should return the used value for width (after sibling removal)");
+}, "getComputedStyle() should return the correct used value for nodes that have had an IB-split sibling removed");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-layout-dependent-replaced-into-ib-split.html b/testing/web-platform/tests/css/cssom/getComputedStyle-layout-dependent-replaced-into-ib-split.html
new file mode 100644
index 0000000000..98771cebc7
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-layout-dependent-replaced-into-ib-split.html
@@ -0,0 +1,30 @@
+<!doctype html>
+<title>getComputedStyle() returns the right style for layout-dependent properties for nodes that have been just inserted into the document, and that have an ancestor whose layout tree was recreated (like an IB-split)</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1585882">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style>
+  div {
+    width: 100%;
+  }
+</style>
+<span>
+  <div></div>
+</span>
+<script>
+test(() => {
+  let oldDiv = document.querySelector("div");
+  window.unused = oldDiv.getBoundingClientRect(); // update layout
+
+  let oldWidth = getComputedStyle(oldDiv).width;
+  assert_true(oldWidth.indexOf("px") !== -1, "Should return the used value for width");
+
+  let newDiv = document.createElement("div");
+  oldDiv.replaceWith(newDiv);
+
+  assert_equals(getComputedStyle(newDiv).width, oldWidth, "Should return the used value for width (just inserted into the document)");
+}, "getComputedStyle() should return used value correctly for nodes just inserted into the document, even if they're in an IB-split");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-line-height.html b/testing/web-platform/tests/css/cssom/getComputedStyle-line-height.html
new file mode 100644
index 0000000000..089a6d8485
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-line-height.html
@@ -0,0 +1,23 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: line-height resolved value</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-values">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/3749">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  div { font-size: 16px; }
+</style>
+<div style="line-height: normal" data-expected="normal"></div>
+<div style="line-height: 1" data-expected="16px"></div>
+<div style="line-height: 10px" data-expected="10px"></div>
+<div style="line-height: 10%" data-expected="1.6px"></div>
+<script>
+for (const e of document.querySelectorAll("div")) {
+  const specified = e.style.lineHeight;
+  test(function() {
+    const expected = e.getAttribute("data-expected");
+    assert_equals(getComputedStyle(e).lineHeight, expected, specified + " should compute to " + expected);
+  }, "line-height: " + specified);
+}
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-logical-enumeration.html b/testing/web-platform/tests/css/cssom/getComputedStyle-logical-enumeration.html
new file mode 100644
index 0000000000..02093120f2
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-logical-enumeration.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: getComputedStyle enumeration</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://bugs.webkit.org/show_bug.cgi?id=210695">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1072180">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script>
+  test(function() {
+    let longhand = false;
+    let shorthand = false;
+    let env = false;
+    for (let property of getComputedStyle(document.documentElement)) {
+      if (property == "block-size")
+        longhand = true;
+      if (property == "padding-block")
+        shorthand = true;
+      if (property == "safe-area-inset-top")
+        env = true;
+    }
+    assert_true(longhand, "Logical longhands should be enumerated in computed style");
+    assert_false(shorthand, "Logical shorthands should not be enumerated in computed style");
+    assert_false(env, "Environment variables should not be enumerated in computed style");
+  }, "Logical properties in enumeration of computed style")
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-property-order.html b/testing/web-platform/tests/css/cssom/getComputedStyle-property-order.html
new file mode 100644
index 0000000000..f52700eb94
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-property-order.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM: getComputedStyle property order</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<meta name="assert" content="This test checks that in a computed style, properties are sorted in lexicographical order." />
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script>
+test(function() {
+  const properties = [...getComputedStyle(document.documentElement)];
+  const sorted = properties.slice().sort(function(a,b) {
+    if (a === b) {
+      assert_unreached("There shouldn't be repeated properties");
+      return 0;
+    }
+    // Sort vendor-prefixed properties after standard ones
+    if (a.startsWith("-") != b.startsWith("-")) {
+      return b.startsWith("-") ? -1 : 1;
+    }
+    // Sort with lexicographical order
+    return a < b ? -1 : 1;
+  });
+  assert_array_equals(properties, sorted);
+}, "Computed style properties should be sorted lexicographically");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-pseudo.html b/testing/web-platform/tests/css/cssom/getComputedStyle-pseudo.html
new file mode 100644
index 0000000000..40af5b0818
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-pseudo.html
@@ -0,0 +1,153 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: Correct resolution of resolved value for display-affected pseudo-elements</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-values">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<style>
+#test { width: 100px; }
+
+#contents {
+  display: contents;
+  border: 10px solid red;
+}
+
+#test::before,
+#test::after,
+#contents::before,
+#contents::after,
+#flex::before,
+#flex::after {
+  content: " ";
+  width: 50%;
+  height: 10px;
+  display: block;
+}
+#none {
+  display: none;
+}
+#none::before,
+#none::after {
+  content: "Foo";
+}
+#flex {
+  display: flex;
+}
+#flex-no-pseudo {
+  display: flex;
+}
+#contents-pseudos::before,
+#contents-pseudos::after {
+  display: contents;
+  content: "foo";
+  position: absolute;
+}
+#contents-pseudos-dynamic::before,
+#contents-pseudos-dynamic::after {
+  display: block;
+  content: "foo";
+  position: absolute;
+}
+#contents-pseudos-dynamic.contents::before,
+#contents-pseudos-dynamic.contents::after {
+  display: contents;
+}
+</style>
+<div id="test">
+  <div id="contents"></div>
+  <div id="none"></div>
+  <div id="flex"></div>
+  <div id="flex-no-pseudo"></div>
+  <div id="contents-pseudos"></div>
+  <div id="contents-pseudos-dynamic"></div>
+</div>
+<script>
+test(function() {
+  var div = document.getElementById('test');
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(div, pseudo).width, "50px");
+  });
+}, "Resolution of width is correct for ::before and ::after pseudo-elements");
+test(function() {
+  var contents = document.getElementById('contents');
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(contents, pseudo).width, "50px");
+  });
+}, "Resolution of width is correct for ::before and ::after pseudo-elements of display: contents elements");
+test(function() {
+  var has_no_pseudos = document.body;
+  has_no_pseudos.style.position = "relative";
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(has_no_pseudos, pseudo).position, "static",
+                  "Nonexistent " + pseudo + " pseudo-element shouldn't claim to have " +
+                  "the same style as the originating element");
+    assert_equals(getComputedStyle(has_no_pseudos, pseudo).width, "auto",
+                  "Nonexistent " + pseudo + " pseudo-element shouldn't claim to have " +
+                  "definite size");
+  });
+}, "Resolution of nonexistent pseudo-element styles");
+test(function() {
+  var none = document.getElementById('none');
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(none, pseudo).content, "\"Foo\"",
+                  "Pseudo-styles of display: none elements should be correct");
+  });
+}, "Resolution of pseudo-element styles in display: none elements");
+test(function() {
+  var flex = document.getElementById('flex');
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(flex, pseudo).display, "block",
+                  "Pseudo-styles of display: flex elements should get blockified");
+  });
+}, "Item-based blockification of pseudo-elements");
+test(function() {
+  var flexNoPseudo = document.getElementById('flex-no-pseudo');
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(flexNoPseudo, pseudo).display, "block",
+                  "Pseudo-styles of display: flex elements should get blockified");
+  });
+}, "Item-based blockification of nonexistent pseudo-elements");
+test(function() {
+  var contentsPseudos = document.getElementById('contents-pseudos');
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(contentsPseudos, pseudo).display, "contents",
+                  "display: contents in " + pseudo + " should get reflected on CSSOM");
+    assert_equals(getComputedStyle(contentsPseudos, pseudo).width, "auto",
+                  pseudo + " with display: contents should have no box");
+    assert_equals(getComputedStyle(contentsPseudos, pseudo).position, "absolute",
+                  "display: contents in " + pseudo + " should reflect other non-inherited properties in CSSOM");
+  });
+}, "display: contents on pseudo-elements");
+test(function() {
+  var contentsPseudosDynamic = document.getElementById('contents-pseudos-dynamic');
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(contentsPseudosDynamic, pseudo).display, "block",
+                  "Check that display for " + pseudo + " is block before change");
+  });
+  contentsPseudosDynamic.className = "contents";
+  [":before", ":after"].forEach(function(pseudo) {
+    assert_equals(getComputedStyle(contentsPseudosDynamic, pseudo).display, "contents",
+                  "display: contents in " + pseudo + " should get reflected on CSSOM");
+    assert_equals(getComputedStyle(contentsPseudosDynamic, pseudo).width, "auto",
+                  pseudo + " with display: contents should have no box");
+    assert_equals(getComputedStyle(contentsPseudosDynamic, pseudo).position, "absolute",
+                  "display: contents in " + pseudo + " should reflect other non-inherited properties in CSSOM");
+  });
+}, "Dynamically change to display: contents on pseudo-elements");
+test(function() {
+  var div = document.getElementById('test');
+  // Note that these assertions deliberately avoid assert_[not_]equals to
+  // avoid gCS().length in the failure output.
+  assert_true(
+    getComputedStyle(div, "totallynotapseudo").length != 0,
+    "Should return the element's style for unknown pseudo-elements that don't start with a colon");
+  assert_true(
+    getComputedStyle(div, "::totallynotapseudo").length == 0,
+    "Should return an empty style for unknown pseudo-elements starting with double-colon");
+  assert_true(
+    getComputedStyle(div, ":totallynotapseudo").length == 0,
+    "Should return an empty style for unknown pseudo-elements starting with colon");
+}, "Unknown pseudo-elements");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-resolved-colors.html b/testing/web-platform/tests/css/cssom/getComputedStyle-resolved-colors.html
new file mode 100644
index 0000000000..da4fddbc24
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-resolved-colors.html
@@ -0,0 +1,42 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of certain color properties are used values</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  #target {
+    background-color: Menu;
+    border: 1px solid Menu;
+    box-shadow: 1px 1px Menu;
+    caret-color: Menu;
+    color: Menu;
+    outline-color: Menu;
+  }
+</style>
+<div id="target"></div>
+<script>
+  const properties_to_test = [
+    "background-color",
+    "border-block-end-color",
+    "border-block-start-color",
+    "border-bottom-color",
+    "border-inline-end-color",
+    "border-inline-start-color",
+    "border-left-color",
+    "border-right-color",
+    "border-top-color",
+    "box-shadow",
+    "caret-color",
+    "color",
+    "outline-color",
+  ];
+
+  for (const property of properties_to_test) {
+    test(function() {
+      let resolved_value =
+        window.getComputedStyle(document.getElementById("target")).getPropertyValue(property);
+      assert_regexp_match(resolved_value, /^rgb[a]?\(/);
+    }, "The resolved value for '" + property + "' is the used value");
+  }
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-resolved-min-max-clamping.html b/testing/web-platform/tests/css/cssom/getComputedStyle-resolved-min-max-clamping.html
new file mode 100644
index 0000000000..e630377c33
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-resolved-min-max-clamping.html
@@ -0,0 +1,38 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM: resolved values of the width and height when the element generates no box or are a non-replaced inline aren't clamped by min-width / max-width</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<span id="non-replaced-inline"></span>
+<div id="display-none" style="display: none"></div>
+<div id="display-contents" style="display: contents"></div>
+<script>
+test(function() {
+  for (const e of document.querySelectorAll("[id]")) {
+    const cs = getComputedStyle(e);
+    for (const prop of ["width", "height"]) {
+      e.style.setProperty("min-" + prop, "10px");
+      e.style.setProperty("max-" + prop, "50px");
+
+      e.style.setProperty(prop, "10%");
+      assert_equals(cs[prop], "10%", `${e.id}: ${prop} with percentages returns percentages`);
+
+      e.style.setProperty(prop, "15px");
+      assert_equals(cs[prop], "15px", `${e.id}: ${prop} with value in range returns computed value`);
+
+      e.style.setProperty(prop, "1px");
+      assert_equals(cs[prop], "1px", `${e.id}: ${prop} with value out of range isn't clamped by min-${prop}`);
+
+      e.style.setProperty(prop, "60px");
+      assert_equals(cs[prop], "60px", `${e.id}: ${prop} with value out of range isn't clamped by max-${prop}`);
+
+      e.style.removeProperty(prop);
+      e.style.removeProperty("min-" + prop);
+      e.style.removeProperty("max-" + prop);
+    }
+  }
+}, "Resolved value of width / height when there's no used value isn't clamped by min/max properties");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-sticky-pos-percent.html b/testing/web-platform/tests/css/cssom/getComputedStyle-sticky-pos-percent.html
new file mode 100644
index 0000000000..12ad5e8965
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-sticky-pos-percent.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<title>CSS Test: resolve top percentage value against proper box</title>
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/3115" />
+<link rel="help" href="https://drafts.csswg.org/css-position/#sticky-pos" />
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-values" />
+<meta name="assert" content="Test that the sticky percentage insets are
+resolved against the right ancestor, i.e. the nearest scrollport." />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<div style="height: 500px; background: purple; overflow: hidden;">
+  <div style="height: 400px; background: yellow;">
+    <div id="target" style="height: 100px; position: sticky; left: 0; top: 50%; background: blue;">
+    </div>
+  </div>
+</div>
+<script>
+test(() => {
+  assert_equals(getComputedStyle(target).top, '250px');
+}, "Sticky element's top property percentage value should be resolved against the div with overflow: hidden");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/getComputedStyle-width-scroll.tentative.html b/testing/web-platform/tests/css/cssom/getComputedStyle-width-scroll.tentative.html
new file mode 100644
index 0000000000..8f629c7016
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/getComputedStyle-width-scroll.tentative.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<title>getComputedStyle() round-trips in presence of scrollbars.</title>
+<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel="author" href="https://mozilla.org" title="Mozilla">
+<link rel="help" href="https://drafts.csswg.org/cssom/#resolved-value">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<div style="width: 100px; height: 100px; overflow: scroll"></div>
+<script>
+// NOTE(emilio): This is a .tentative.html because the spec is not clear on
+// what the used value of `width` or `height` is, but I think this behavior
+// should be uncontroversial.
+test(function() {
+  let e = document.querySelector("div");
+  let cs = getComputedStyle(e);
+
+  let originalWidth = cs.width;
+  let originalHeight = cs.height;
+
+  e.style.width = originalWidth;
+  e.style.height = originalHeight;
+
+  assert_equals(cs.width, originalWidth, "width round-trips");
+  assert_equals(cs.height, originalHeight, "height round-trips");
+}, "getComputedStyle() round-trips in presence of scrollbars")
+</script>
diff --git a/testing/web-platform/tests/css/cssom/historical.html b/testing/web-platform/tests/css/cssom/historical.html
new file mode 100644
index 0000000000..44f3f6c8f1
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/historical.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<title>Historical features</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#changes-from-5-december-2013">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id=log></div>
+<script>
+[
+  "selectedStyleSheetSet",
+  "lastStyleSheetSet",
+  "preferredStyleSheetSet",
+  "styleSheetSets",
+  "enableStyleSheetsForSet",
+  "selectedStylesheetSet",
+  "preferredStylesheetSet",
+].forEach(function(name) {
+  test(function() {
+    assert_false(name in document);
+
+    var doc = document.implementation.createDocument(null, null, null);
+    assert_false(name in doc);
+  }, "Historical Document member: " + name);
+});
+
+[
+  "Rect",
+  "RGBColor",
+  "CSSValue",
+  "CSSPrimitiveValue",
+  "CSSValueList",
+].forEach(function(name) {
+  test(function() {
+    assert_false(name in window);
+  }, "Historical interface: " + name);
+});
+
+[
+  "getPropertyCSSValue",
+  "setPropertyValue",
+  "setPropertyPriority",
+].forEach(function(name) {
+  test(function() {
+    assert_false(name in document.body.style);
+  }, "Historical CSSStyleDeclaration member: " + name);
+});
+
+[
+  "cascadedStyle",
+  "defaultStyle",
+  "rawComputedStyle",
+  "usedStyle",
+].forEach(function(name) {
+  test(function() {
+    assert_false(name in document.body);
+    assert_false(name in document.createElement("test"));
+  }, "Historical Element member: " + name);
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/idlharness.html b/testing/web-platform/tests/css/cssom/idlharness.html
new file mode 100644
index 0000000000..e27629c03b
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/idlharness.html
@@ -0,0 +1,77 @@
+<!doctype html>
+<meta charset=utf-8>
+<!-- WARNING: These tests are preliminary and probably partly incorrect.  -->
+<title>CSSOM automated IDL tests</title>
+<link rel="author" title="Ms2ger" href="mailto:Ms2ger@gmail.com">
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#idl-index">
+<meta name="timeout" content="long">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script src=/resources/WebIDLParser.js></script>
+<script src=/resources/idlharness.js></script>
+<!--
+Provide some objects to test.
+Use a non-empty style attribute to get a non-empty CSSStyleDeclaration.
+-->
+<style id="styleElement" style="z-index: 0;">
+@import url("data:text/css,");
+@namespace x "y";
+@page { @top-left {} }
+@media all {}
+#test { color: green; }
+</style>
+
+<svg id="svgElement" style="height: 0;"></svg>
+
+<iframe id="xmlssPiIframe" src="support/xmlss-pi.xhtml" style="display: none;"></iframe>
+
+<h1>CSSOM IDL tests</h1>
+<div id=log></div>
+
+<script>
+'use strict';
+
+const waitForLoad = new Promise(r => { addEventListener('load', r); });
+
+idl_test(
+  ['cssom'],
+  ['SVG', 'uievents', 'html', 'dom', 'mathml-core'],
+  async idlArray => {
+    idlArray.add_objects({
+      Document: ['document', 'new Document()'],
+      StyleSheetList: ['document.styleSheets'],
+      CSSStyleSheet: ['sheet'],
+      MediaList: ['sheet.media'],
+      CSSRuleList: ['sheet.cssRules'],
+      CSSImportRule: ['sheet.cssRules[0]'],
+      CSSNamespaceRule: ['sheet.cssRules[1]'],
+      CSSPageRule: ['sheet.cssRules[2]'],
+      CSSMarginRule: ['sheet.cssRules[2].cssRules[0]'],
+      CSSMediaRule: ['sheet.cssRules[3]'],
+      CSSStyleRule: ['sheet.cssRules[4]'],
+      CSSStyleDeclaration: [
+        'sheet.cssRules[4].style', // CSSStyleRule
+        'sheet.cssRules[2].style', // CSSPageRule
+        'sheet.cssRules[2].cssRules[0].style', // CSSMarginRule
+        'style_element.style', // ElementCSSInlineStyle for HTMLElement
+        'svg_element.style', // ElementCSSInlineStyle for SVGElement
+        'getComputedStyle(svg_element)'
+      ],
+      ProcessingInstruction: ['xmlss_pi'],
+      Window: ['window'],
+      HTMLElement: [
+        'style_element',
+        'document.createElement("unknownelement")'
+      ],
+      SVGElement: ['svg_element'],
+    });
+
+    await waitForLoad;
+    self.style_element = document.getElementById('styleElement');
+    self.sheet = style_element.sheet;
+    self.svg_element = document.getElementById('svgElement');
+    self.xmlss_pi = document.getElementById('xmlssPiIframe').contentDocument.firstChild;
+  }
+);
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/inline-style-001.html b/testing/web-platform/tests/css/cssom/inline-style-001.html
new file mode 100644
index 0000000000..d4bf3b92ab
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/inline-style-001.html
@@ -0,0 +1,78 @@
+<!DOCTYPE html>
+<html>
+ <head>
+  <title>CSS Test: Inline CSSStyleDeclaration</title>
+  <link rel="author" title="Bear Travis" href="mailto:betravis@adobe.com">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#elementcssinlinestyle">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="Inline CSSStyleDeclaration is properly initialized and can be modified through its interface">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+ </head>
+ <body>
+ <noscript>Test not run - javascript required.</noscript>
+ <div id="log"></div>
+ <div id="test" style="margin-left: 5px;"></div>
+ <script type="text/javascript">
+    // Can access CSSStyleDeclaration through style property
+    test(function() {
+        var test = document.getElementById("test");
+        assert_idl_attribute(test, "style");
+        declaration = test.style;
+    }, "CSSStyleDeclaration_accessible");
+
+    // initial property values are correct
+    test(function() {
+        assert_equals(declaration.cssText, "margin-left: 5px;");
+        assert_equals(declaration.getPropertyValue("margin-left"), "5px");
+    }, "read");
+
+    // setting cssText adds new properties
+    // setting cssText removes existing properties
+    // properties set through cssText are reflected in the computed style
+    test(function() {
+        declaration.cssText = "margin-left: 10px; padding-left: 10px;";
+        assert_equals(declaration.cssText, "margin-left: 10px; padding-left: 10px;");
+        assert_equals(declaration.length, 2);
+        assert_equals(declaration.item(0), "margin-left");
+        assert_equals(declaration.item(1), "padding-left");
+        assert_equals(declaration.getPropertyValue("margin-left"), "10px");
+        assert_equals(declaration.getPropertyValue("padding-left"), "10px");
+
+        var computedStyle = window.getComputedStyle(document.getElementById("test"));
+        assert_equals(computedStyle.getPropertyValue("margin-left"), "10px");
+        assert_equals(computedStyle.getPropertyValue("padding-left"), "10px");
+    }, "csstext_write");
+
+    // setProperty adds new properties
+    // properties set through setProperty are reflected in the computed style
+    test(function() {
+        while(declaration.length > 0)
+            declaration.removeProperty(declaration.item(0));
+        declaration.setProperty("margin-left", "15px");
+        declaration.setProperty("padding-left", "15px");
+
+        assert_equals(declaration.length, 2);
+        assert_equals(declaration.item(0), "margin-left");
+        assert_equals(declaration.item(1), "padding-left");
+        assert_equals(declaration.getPropertyValue("margin-left"), "15px");
+        assert_equals(declaration.getPropertyValue("padding-left"), "15px");
+
+        var computedStyle = window.getComputedStyle(document.getElementById("test"));
+        assert_equals(computedStyle.getPropertyValue("margin-left"), "15px");
+        assert_equals(computedStyle.getPropertyValue("padding-left"), "15px");
+    }, "property_write");
+
+    // shorthand property is expanded
+    test(function() {
+        while(declaration.length > 0)
+            declaration.removeProperty(declaration.item(0));
+        declaration.cssText = "margin: 20px";
+        assert_equals(declaration.getPropertyValue("margin-top"), "20px");
+        assert_equals(declaration.getPropertyValue("margin-right"), "20px");
+        assert_equals(declaration.getPropertyValue("margin-bottom"), "20px");
+        assert_equals(declaration.getPropertyValue("margin-left"), "20px");
+    }, "shorthand_properties");
+ </script>
+ </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/insertRule-across-context.html b/testing/web-platform/tests/css/cssom/insertRule-across-context.html
new file mode 100644
index 0000000000..a8327e31ac
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-across-context.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS Test: CSSOM StyleSheet insertRule across context</title>
+<link rel="author" title="Kagami Sascha Rosylight" href="mailto:saschanaz@outlook.com">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://drafts.csswg.org/cssom/">
+<script src="/resources/testharness.js" type="text/javascript"></script>
+<script src="/resources/testharnessreport.js" type="text/javascript"></script>
+
+<iframe id="iframe"></iframe>
+<script>
+  function createSheet() {
+    const tempStyleElement = iframe.contentDocument.createElement('style');
+    iframe.contentDocument.head.append(tempStyleElement);
+
+    const tempStyle = tempStyleElement.sheet;
+    tempStyleElement.remove();
+    return tempStyle;
+  }
+
+  test(() => {
+    const sheet = createSheet();
+    sheet.insertRule(".kaoru {}");
+    assert_equals(sheet.rules[0].constructor, iframe.contentWindow.CSSStyleRule);
+  }, "The constructor of inserted rule object must be from iframe");
+
+  test(() => {
+    const sheet = new iframe.contentWindow.CSSStyleSheet();
+    sheet.insertRule(".kaoru {}");
+    assert_equals(sheet.rules[0].constructor, iframe.contentWindow.CSSStyleRule);
+  }, "The constructor of inserted rule object must be from iframe for new CSSStyleSheet()");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/insertRule-charset-no-index.html b/testing/web-platform/tests/css/cssom/insertRule-charset-no-index.html
new file mode 100644
index 0000000000..16f2358dc2
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-charset-no-index.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSS Test: CSSOM StyleSheet insertRule with charset and omitted second argument</title>
+    <link rel="author" title="Sendil Kumar N" href="mailto:sendilkumarn.opensource@gmail.com">
+    <link rel="help" href="https://drafts.csswg.org/cssom/">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+    <meta name="flags" content="dom">
+    <script src="/resources/testharness.js" type="text/javascript"></script>
+    <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+    <link rel="stylesheet" type="text/css" href="support/import-charset.css" id="linkElement" >
+</head>
+<body>
+<div id="log"></div>
+<script type="text/javascript">
+    var sheet = document.getElementById("linkElement").sheet;
+    test(function() {
+        assert_equals(sheet.cssRules.length, 0);
+        sheet.insertRule("p { color: green; }");
+        assert_equals(sheet.cssRules.length, 1);
+        assert_equals(sheet.cssRules.item(0).cssText, "p { color: green; }");
+    }, "insertRule with charset and omitted index argument");
+
+    test(function() {
+        assert_equals(sheet.cssRules.length, 1);
+        sheet.insertRule("p { color: yellow; }", undefined);
+        assert_equals(sheet.cssRules.length, 2);
+        assert_equals(sheet.cssRules.item(0).cssText, "p { color: yellow; }");
+    }, "insertRule with charset and undefined index argument");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/insertRule-from-script-ref.html b/testing/web-platform/tests/css/cssom/insertRule-from-script-ref.html
new file mode 100644
index 0000000000..2fa45526a6
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-from-script-ref.html
@@ -0,0 +1,6 @@
+<!doctype html>
+<head>
+  <meta charset="utf-8">
+  <title>insertrule @import test reference</title>
+  <link rel="stylesheet" href="support/black.css">
+</head>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/insertRule-from-script.html b/testing/web-platform/tests/css/cssom/insertRule-from-script.html
new file mode 100644
index 0000000000..c364afc282
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-from-script.html
@@ -0,0 +1,12 @@
+<!doctype html>
+<head>
+  <meta charset="utf-8">
+  <title>insertrule @import test</title>
+  <link rel="help" href="https://drafts.csswg.org/cssom/">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+  <link rel="match" href="insertRule-from-script-ref.html">
+  <style></style>
+</head>
+<body>
+  <script>document.styleSheets[0].insertRule("@import url(\"support/black.css\");");</script>
+</body>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/insertRule-import-no-index.html b/testing/web-platform/tests/css/cssom/insertRule-import-no-index.html
new file mode 100644
index 0000000000..45481e71f4
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-import-no-index.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSS Test: CSSOM StyleSheet insertRule with import and omitted second argument</title>
+    <link rel="author" title="Sendil Kumar N" href="mailto:sendilkumarn.opensource@gmail.com">
+    <link rel="help" href="https://drafts.csswg.org/cssom/">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+    <meta name="flags" content="dom">
+    <script src="/resources/testharness.js" type="text/javascript"></script>
+    <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+    <style id="styleElement">
+        @import url("support/a-green.css");
+    </style>
+</head>
+<body>
+<div id="log"></div>
+<script type="text/javascript">
+    var sheet = document.getElementById("styleElement").sheet;
+    test(function() {
+        assert_equals(sheet.cssRules.length, 1);
+        assert_throws_dom("HierarchyRequestError", function() { sheet.insertRule("p { color: green; }"); });
+        assert_equals(sheet.cssRules.length, 1);
+    }, "insertRule with import and omitted index argument");
+
+    test(function() {
+        assert_equals(sheet.cssRules.length, 1);
+        assert_throws_dom("HierarchyRequestError", function() { sheet.insertRule("p { color: yellow; }", undefined); });
+        assert_equals(sheet.cssRules.length, 1);
+        assert_equals(sheet.cssRules.item(0).cssText, "@import url(\"support/a-green.css\");");
+    }, "insertRule with import and undefined index argument");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/insertRule-namespace-no-index.html b/testing/web-platform/tests/css/cssom/insertRule-namespace-no-index.html
new file mode 100644
index 0000000000..3265159736
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-namespace-no-index.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSS Test: CSSOM StyleSheet insertRule with namespace and omitted second argument</title>
+    <link rel="author" title="Sendil Kumar N" href="mailto:sendilkumarn.opensource@gmail.com">
+    <link rel="help" href="https://drafts.csswg.org/cssom/">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+    <meta name="flags" content="dom">
+    <script src="/resources/testharness.js" type="text/javascript"></script>
+    <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+    <style id="styleElement">
+        @namespace svg url(http://servo);
+        @namespace url(http://servo1);
+        @namespace svg url("http://servo2");
+    </style>
+</head>
+<body>
+<div id="log"></div>
+<script type="text/javascript">
+    var sheet = document.getElementById("styleElement").sheet;
+    test(function() {
+        assert_equals(sheet.cssRules.length, 3);
+        assert_throws_dom("HierarchyRequestError", function() { sheet.insertRule("p { color: green; }"); });
+        assert_equals(sheet.cssRules.length, 3);
+    }, "insertRule with namespace and omitted index argument");
+
+    test(function() {
+        assert_equals(sheet.cssRules.length, 3);
+        assert_throws_dom("HierarchyRequestError", function() { sheet.insertRule("p { color: yellow; }", undefined); });
+        assert_equals(sheet.cssRules.length, 3);
+    }, "insertRule with namespace and undefined index argument");
+
+    test(function() {
+        assert_equals(sheet.cssRules.length, 3);
+        sheet.insertRule("@import url(\"support/a-green.css\");");
+        assert_equals(sheet.cssRules.length, 4);
+    }, "insertRule with namespace and omitted index argument should insert import");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/insertRule-no-index.html b/testing/web-platform/tests/css/cssom/insertRule-no-index.html
new file mode 100644
index 0000000000..b4370bd080
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-no-index.html
@@ -0,0 +1,35 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSS Test: CSSOM StyleSheet insertRule omitted second argument</title>
+    <link rel="author" title="Sendil Kumar N" href="mailto:sendilkumarn.opensource@gmail.com">
+    <link rel="help" href="https://drafts.csswg.org/cssom/">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+    <meta name="flags" content="dom">
+    <script src="/resources/testharness.js" type="text/javascript"></script>
+    <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+    <style id="styleElement">
+        /* An initial style rule to test where the new rule is inserted relative to this one */
+        nosuchelement { color: red; }
+    </style>
+</head>
+<body>
+<div id="log"></div>
+<script type="text/javascript">
+    var sheet = document.getElementById("styleElement").sheet;
+    test(function() {
+        assert_equals(sheet.cssRules.length, 1);
+        sheet.insertRule("p { color: green; }");
+        assert_equals(sheet.cssRules.length, 2);
+        assert_equals(sheet.cssRules.item(0).cssText, "p { color: green; }");
+    }, "insertRule with omitted index argument");
+
+    test(function() {
+        assert_equals(sheet.cssRules.length, 2);
+        sheet.insertRule("p { color: yellow; }", undefined);
+        assert_equals(sheet.cssRules.length, 3);
+        assert_equals(sheet.cssRules.item(0).cssText, "p { color: yellow; }");
+    }, "insertRule with undefined index argument");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/insertRule-syntax-error-01.html b/testing/web-platform/tests/css/cssom/insertRule-syntax-error-01.html
new file mode 100644
index 0000000000..4461a9a943
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/insertRule-syntax-error-01.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/cssom-1/#insert-a-css-rule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+p { color: green; }
+</style>
+<p id="test">This text should be green.</p>
+<script>
+    test(function() {
+        assert_throws_dom("SyntaxError", () => document.styleSheets[0].insertRule("p { color: red; } garbage", 1));
+        assert_equals(getComputedStyle(document.getElementById("test")).color, "rgb(0, 128, 0)");
+    }, "A syntax error in insertRule should throw and not affect the style of the page");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/medialist-dynamic-001-ref.html b/testing/web-platform/tests/css/cssom/medialist-dynamic-001-ref.html
new file mode 100644
index 0000000000..9715b5acb8
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/medialist-dynamic-001-ref.html
@@ -0,0 +1,5 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test Reference</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+Should not be red.
diff --git a/testing/web-platform/tests/css/cssom/medialist-dynamic-001.html b/testing/web-platform/tests/css/cssom/medialist-dynamic-001.html
new file mode 100644
index 0000000000..ab9c134568
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/medialist-dynamic-001.html
@@ -0,0 +1,12 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: Dynamic changes to the stylesheet media attributes via CSSOM get reflected</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="match" href="medialist-dynamic-001-ref.html">
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-medialist-mediatext">
+<style media="all">* { color: red }</style>
+Should not be red.
+<script>
+  document.body.offsetTop;
+  document.styleSheets[0].media.mediaText = "not all";
+</script>
diff --git a/testing/web-platform/tests/css/cssom/medialist-interfaces-001.html b/testing/web-platform/tests/css/cssom/medialist-interfaces-001.html
new file mode 100644
index 0000000000..f436177fb8
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/medialist-interfaces-001.html
@@ -0,0 +1,83 @@
+<!DOCTYPE html>
+<html>
+ <head>
+  <title>CSS Test: CSSOM Media Query List Serialization</title>
+  <link rel="author" title="Ben Sheldon" href="mailto:ben@codeforamerica.org">
+  <link rel="author" title="Chapman Shoop" href="mailto:chapman.shoop@gmail.com">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-medialist-interface">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#serializing-media-queries">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#serialize-a-media-query-list">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="MediaLists are serialized according to the specification">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+ </head>
+ <body>
+  <noscript>Test not run - javascript required.</noscript>
+  <div id="log"></div>
+  <script type="text/javascript">
+
+    var styleElement;
+    var styleSheet;
+    var mediaList;
+
+    // Setup
+    function setup() {
+      styleElement = document.getElementById("styleElement");
+
+      if (styleElement) {
+        // teardown
+        document.getElementsByTagName("head")[0].removeChild(styleElement);
+        styleElement = undefined;
+        styleSheet = undefined;
+        mediaList = undefined;
+      }
+
+      styleElement = document.createElement("style");
+      styleElement.id = "styleElement";
+      styleElement.type = "text/css";
+      styleElement.media = "all";
+      document.getElementsByTagName("head")[0].appendChild(styleElement);
+      styleSheet = styleElement.sheet;
+      mediaList = styleSheet.media;
+    }
+
+    // MediaList.mediaText equals the 'media' value of the initial 'style' element.
+    test(function() {
+        setup();
+
+        assert_equals(mediaList.mediaText, "all");
+
+    }, "mediatest_medialist_serialize_element");
+
+    // To serialize a comma-separated list concatenate all items of the list in list order while separating them by \",\" (U+002C), followed by a space (U+0020).
+    test(function() {
+      setup();
+
+      mediaList.appendMedium('screen');
+      assert_equals(mediaList.mediaText, "all, screen");
+
+    }, "mediatest_medialist_serialize_comma");
+
+    // If the media query list is empty return the empty string.
+    test(function() {
+      setup();
+
+      mediaList.deleteMedium('all');
+      assert_equals(mediaList.mediaText, "");
+
+    }, "mediatest_medialist_serialize_empty");
+
+    // Each media query should be sorted in the same order as they appear in the list of media queries.
+    test(function() {
+      setup();
+
+      mediaList.appendMedium('screen');
+      mediaList.appendMedium('print');
+      assert_equals(mediaList.mediaText, "all, screen, print");
+
+    }, "mediatest_medialist_serialize_order");
+
+  </script>
+ </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/medialist-interfaces-002.html b/testing/web-platform/tests/css/cssom/medialist-interfaces-002.html
new file mode 100644
index 0000000000..0d2ca2e9c7
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/medialist-interfaces-002.html
@@ -0,0 +1,71 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <title>CSS Test: CSSOM MediaList Interfaces</title>
+  <link rel="author" title="Chapman Shoop" href="mailto:chapman.shoop@gmail.com">
+  <link rel="reviewer" title="Ms2ger" href="mailto:ms2ger@gmail.com"> <!-- 2012-06-17 -->
+  <link rel="help" href="https://drafts.csswg.org/cssom/#the-medialist-interface">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="MediaList object has deleteMedium method and it functions properly.">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+</head>
+
+<body>
+  <noscript>Test not run - javascript required.</noscript>
+  <div id="log"></div>
+
+  <script type="text/javascript">
+    function setup() {
+      // Clean out any old style element
+      var old_style_el = document.getElementById('test_style');
+      if (old_style_el) {
+        document.head.removeChild(old_style_el);
+      }
+
+      // Create a fresh style element and return its media attribute
+      var style_el = document.createElement('style');
+      style_el.setAttribute('id', 'test_style');
+      document.head.appendChild(style_el);
+      return style_el.sheet.media;
+    }
+  </script>
+
+  <script type="text/javascript">
+
+    // MediaList.deleteMedium called without argument throws error.
+    test(function() {
+      media_list = setup();
+      assert_throws_js(TypeError, function() { media_list.deleteMedium(); });
+    }, "deleteMedium_called_without_argument");
+
+    // MediaList.deleteMedium removes correct medium and updates corresponding properties.
+    test(function() {
+      media_list = setup();
+
+      media_list.appendMedium("screen");
+      media_list.appendMedium("all");
+
+      media_list.deleteMedium("screen");
+
+      assert_equals(media_list.length, 1);
+      assert_equals(media_list.item(0), "all");
+      assert_equals(media_list.mediaText, "all");
+    }, "deleteMedium_removes_correct_medium");
+
+    // MediaList.deleteMedium doesn't modify MediaList when medium is not found.
+    test(function() {
+      media_list = setup();
+
+      media_list.appendMedium("all");
+
+      assert_throws_dom("NotFoundError", () => media_list.deleteMedium("screen"));
+
+      assert_equals(media_list.length, 1);
+      assert_equals(media_list.item(0), "all");
+      assert_equals(media_list.mediaText, "all");
+    }, "deleteMedium_no_matching_medium_to_remove");
+
+  </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/medialist-interfaces-004.html b/testing/web-platform/tests/css/cssom/medialist-interfaces-004.html
new file mode 100644
index 0000000000..7cbea37cba
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/medialist-interfaces-004.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <title>CSS Test: CSSOM MediaList Interfaces</title>
+  <link rel="author" title="Chapman Shoop" href="mailto:chapman.shoop@gmail.com">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-medialist-interface">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="MediaList object has appendMedium method and it functions properly.">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+</head>
+
+<body>
+  <noscript>Test not run - javascript required.</noscript>
+  <div id="log"></div>
+
+  <script type="text/javascript">
+    function setup() {
+      // Clean out any old style element
+      var old_style_el = document.getElementById('test_style');
+      if (old_style_el) {
+        document.head.removeChild(old_style_el);
+      }
+
+      // Create a fresh style element and return its media attribute
+      var style_el = document.createElement('style');
+      style_el.setAttribute('id', 'test_style');
+      document.head.appendChild(style_el);
+      return style_el.sheet.media;
+    }
+  </script>
+
+  <script type="text/javascript">
+
+    // MediaList.appendMedium correctly adds medium to empty MediaList.
+    test(function() {
+      media_list = setup();
+
+      media_list.appendMedium("all");
+
+      assert_equals(media_list.length, 1);
+      assert_equals(media_list.item(0), "all");
+      assert_equals(media_list.mediaText, "all");
+    }, "appendMedium_correctly_appends_medium_to_empty_MediaList");
+
+    // MediaList.appendMedium correctly adds medium to a MediaList that already has a medium.
+    test(function() {
+      media_list = setup();
+
+      media_list.appendMedium("screen");
+      media_list.appendMedium("all");
+
+      // The ordering of array items should be different from that of the mediaText element.
+      // "all" should be appended after "screen" in the array, but "mediaText" should be
+      // "all, screen" due to lexicographical sorting.
+      assert_equals(media_list.length, 2);
+      assert_equals(media_list.item(0), "screen");
+      assert_equals(media_list.item(1), "all");
+      assert_equals(media_list.mediaText, "screen, all");
+    }, "appendMedium_correctly_appends_medium_to_nonempty_MediaList");
+
+  </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/mediaquery-sort-dedup.html b/testing/web-platform/tests/css/cssom/mediaquery-sort-dedup.html
new file mode 100644
index 0000000000..0ae78d0300
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/mediaquery-sort-dedup.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM test: Media query serialization quirks</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#serializing-media-queries">
+<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel="author" href="https://mozilla.org" title="Mozilla">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+test(function() {
+  assert_equals(matchMedia("(min-width: 10px) and (min-height: 10px)").media, "(min-width: 10px) and (min-height: 10px)");
+} , "Media queries are not sorted lexicographically");
+test(function() {
+  assert_equals(matchMedia("(color) and (color)").media, "(color) and (color)");
+}, "Media queries are not deduplicated");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/overflow-serialization.html b/testing/web-platform/tests/css/cssom/overflow-serialization.html
new file mode 100644
index 0000000000..2e4bd97d5c
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/overflow-serialization.html
@@ -0,0 +1,61 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - Overflow shorthand serialization</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-value">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <style>
+        div { overflow: inherit; }
+        div { overflow: hidden; }
+        div { overflow-x: initial; overflow-y: initial; }
+        div { overflow-x: scroll; overflow-y: scroll; }
+        div { overflow-x: scroll; overflow-y: hidden; }
+    </style>
+
+    <script>
+    var styleSheet = document.styleSheets[0]
+    var div = document.createElement('div')
+    test(function () {
+        assert_equals(styleSheet.cssRules[0].style.cssText, "overflow: inherit;")
+    }, "Single value overflow with CSS-wide keyword should serialize correctly.")
+    test(function () {
+        assert_equals(styleSheet.cssRules[1].style.cssText, "overflow: hidden;")
+    }, "Single value overflow with non-CSS-wide keyword should serialize correctly.")
+    test(function () {
+        assert_equals(styleSheet.cssRules[2].style.cssText, "overflow: initial;")
+    }, "Overflow-x/y longhands with same CSS-wide keyword should serialize correctly.")
+    test(function () {
+        assert_equals(styleSheet.cssRules[3].style.cssText, "overflow: scroll;")
+    }, "Overflow-x/y longhands with same non-CSS-wide keyword should serialize correctly.")
+    test(function () {
+        assert_equals(styleSheet.cssRules[4].style.cssText, "overflow: scroll hidden;")
+    }, "Overflow-x/y longhands with different keywords should serialize correctly.")
+    test(function () {
+        div.style.overflow = "inherit"
+        assert_equals(div.style.overflow, "inherit")
+    }, "Single value overflow on element with CSS-wide keyword should serialize correctly.")
+    test(function () {
+        div.style.overflow = "hidden"
+        assert_equals(div.style.overflow, "hidden")
+    }, "Single value overflow on element with non-CSS-wide keyword should serialize correctly.")
+    test(function () {
+        div.style.overflow = ""
+        div.style.overflowX = "initial"
+        div.style.overflowY = "initial"
+        assert_equals(div.style.overflow, "initial")
+    }, "Overflow-x/y longhands on element with same CSS-wide keyword should serialize correctly.")
+    test(function () {
+        div.style.overflowX = "scroll"
+        div.style.overflowY = "scroll"
+        assert_equals(div.style.overflow, "scroll")
+    }, "Overflow-x/y longhands on element with same non-CSS-wide keyword should serialize correctly.")
+    test(function () {
+        div.style.overflowX = "scroll"
+        div.style.overflowY = "hidden"
+        assert_equals(div.style.overflow, "scroll hidden")
+    }, "Overflow-x/y longhands on element with different keywords should serialize correctly.")
+    </script>
+</head>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/page-descriptors.html b/testing/web-platform/tests/css/cssom/page-descriptors.html
new file mode 100644
index 0000000000..5cf0a6a703
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/page-descriptors.html
@@ -0,0 +1,40 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <link rel="author" title="Mozilla" href="https://mozilla.org">
+  <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface">
+  <title>Page descriptors shouldn't be exposed to CSS Style declarations</title>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<div id="target"></div>
+<script>
+'use strict';
+
+let element = document.getElementById("target");
+let computedStyle = window.getComputedStyle(element);
+let style = element.style;
+
+test(t => {
+    assert_equals(computedStyle.size, undefined,
+        "computed style should not have size property");
+    assert_equals(computedStyle.getPropertyValue("size"), "",
+        "computed style getPropertyValue(\"size\") should be empty");
+
+    assert_equals(style.size, undefined,
+        "style should not have size property");
+    assert_equals(style.getPropertyValue("size"), "",
+        "style getPropertyValue(\"size\") should be empty");
+    for(const val of ["initial", "auto", "100px"]){
+        style.setProperty("size", val);
+        assert_false(CSS.supports("size", val));
+        assert_equals(style.size, undefined,
+            "style should not have size property after assigning size=" + val);
+        assert_equals(style.getPropertyValue("size"), "",
+            "style getPropertyValue(\"size\") should be empty after assigning size=" + val);
+    }
+});
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/preferred-stylesheet-order.html b/testing/web-platform/tests/css/cssom/preferred-stylesheet-order.html
new file mode 100644
index 0000000000..dc990131ab
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/preferred-stylesheet-order.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/cssom/#add-a-css-style-sheet">
+<link rel="help" href="https://drafts.csswg.org/cssom/#create-a-css-style-sheet">
+<link rel="help" href="https://html.spec.whatwg.org/#update-a-style-block">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="t1">This text should be green</div>
+<script>
+function createStyleElement(text, title) {
+    var elm = document.createElement("style");
+    elm.setAttribute("title", title);
+    elm.appendChild(document.createTextNode(text));
+    return elm;
+}
+
+test(function() {
+    document.head.appendChild(createStyleElement("#t1 {color:green}", "preferred"));
+    document.head.appendChild(createStyleElement("#t1 {color:red}", "notpreferred"));
+
+    assert_equals(getComputedStyle(t1).color, "rgb(0, 128, 0)");
+}, "Preferred stylesheet where insertion order is reversed tree order");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/preferred-stylesheet-reversed-order.html b/testing/web-platform/tests/css/cssom/preferred-stylesheet-reversed-order.html
new file mode 100644
index 0000000000..ff3a8b0901
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/preferred-stylesheet-reversed-order.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/cssom/#add-a-css-style-sheet">
+<link rel="help" href="https://drafts.csswg.org/cssom/#create-a-css-style-sheet">
+<link rel="help" href="https://html.spec.whatwg.org/#update-a-style-block">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="t1">This text should be green</div>
+<script>
+function createStyleElement(text, title) {
+    var elm = document.createElement("style");
+    elm.setAttribute("title", title);
+    elm.appendChild(document.createTextNode(text));
+    return elm;
+}
+
+test(function() {
+    document.head.insertBefore(createStyleElement("#t1 {color:green}", "preferred"), document.head.firstChild);
+    document.head.insertBefore(createStyleElement("#t1 {color:red}", "notpreferred"), document.head.firstChild);
+
+    assert_equals(getComputedStyle(t1).color, "rgb(0, 128, 0)");
+}, "Preferred stylesheet where insertion order is tree order");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/property-accessors.html b/testing/web-platform/tests/css/cssom/property-accessors.html
new file mode 100644
index 0000000000..87c9df0b32
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/property-accessors.html
@@ -0,0 +1,64 @@
+<!doctype html>
+<html>
+<meta charset="utf-8">
+<title>Accessing properties via CSSStyleDeclaration</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface">
+<!-- this is really a crash test, but let's claim it's a testharness test -->
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+@font-face {}
+</style>
+<div id="testElement"></div>
+<script>
+// Goal here is to test a cross-section of prefixed, properties, and descriptors.
+const properties = [
+  "-apple-color-filter", "-apple-pay-button-style", "-epub-writing-mode",
+  "-webkit-flex", "gap", "grid-gap", "overscroll-behavior",
+  "src", "unicode-range",
+];
+
+const el = document.getElementById("testElement");
+const decls = [window.getComputedStyle(el), el.style, document.styleSheets[0].cssRules[0].style];
+
+for (const prop of properties) {
+  test(() => {
+    for (const decl of decls) {
+      let _;
+
+      _ = decl[prop];
+      try {
+        decl[prop] = "nonsense";
+      } catch {
+        assert_equals(decl, decls[0]);
+      }
+
+      _ = decl.cssText;
+      try {
+        decl.cssText = `${prop}: nonsense`;
+      } catch {
+        assert_equals(decl, decls[0]);
+      }
+
+      _ = decl.getPropertyValue(prop);
+      _ = decl.getPropertyPriority(prop);
+
+      if ("getPropertyCSSValue" in decl) {
+        _ = decl.getPropertyCSSValue(prop);
+      }
+
+      try {
+        decl.setProperty(prop, "nonsense", "");
+      } catch {
+        assert_equals(decl, decls[0]);
+      }
+
+      try {
+        decl.removeProperty(prop);
+      } catch {
+        assert_equals(decl, decls[0]);
+      }
+    }
+  }, prop);
+}
+</script>
diff --git a/testing/web-platform/tests/css/cssom/removerule-invalidation-crash.html b/testing/web-platform/tests/css/cssom/removerule-invalidation-crash.html
new file mode 100644
index 0000000000..a83f43d5fa
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/removerule-invalidation-crash.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<meta charset="UTF-8">
+<title>CSSOM - removeRule doesn't assert when removing a particular set of rules</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#cssstylesheet">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1659718">
+<style>
+@media all {
+  * { color: red; }
+
+  #foobar { color: blue }
+}
+</style>
+<script>
+  document.documentElement.getBoundingClientRect();
+  document.styleSheets[0].deleteRule(0);
+</script>
diff --git a/testing/web-platform/tests/css/cssom/rule-restrictions.html b/testing/web-platform/tests/css/cssom/rule-restrictions.html
new file mode 100644
index 0000000000..ce2dee37a0
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/rule-restrictions.html
@@ -0,0 +1,51 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM Should correctly honor property restrictions</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1665816">
+<link rel="help" href="https://drafts.csswg.org/css-page-3/#conform-partial">
+<link rel="help" href="https://drafts.csswg.org/css-animations/#keyframes">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  @page {
+    margin-top: 10px;
+    transform: scale(1);
+  }
+
+  @keyframes foo {
+    from {
+      margin-top: 10px;
+      animation-name: none;
+    }
+  }
+</style>
+<script>
+test(function() {
+  let rule = document.styleSheets[0].cssRules[0];
+  assert_equals(rule.type, CSSRule.PAGE_RULE, "Should be a page rule");
+  assert_equals(rule.style.length, 1, "Transform doesn't quite apply to pages");
+  assert_equals(rule.style.marginTop, "10px", "Should have a margin-top declaration");
+  rule.style.setProperty("transform", "scale(1)");
+  assert_equals(rule.style.getPropertyValue("transform"), "", "Shouldn't have been able to set the transform property via setProperty");
+  assert_equals(rule.style.length, 1, "Shouldn't have been able to set transform via setProperty");
+  rule.style.cssText = "margin-bottom: 10px; transform: scale(1);";
+  assert_equals(rule.style.length, 1, "Should not have been able to set transform via cssText");
+  assert_equals(rule.style.marginBottom, "10px", "Should have a margin-bottom declaration");
+}, "@page");
+
+test(function() {
+  let rule = document.styleSheets[0].cssRules[1].cssRules[0];
+  assert_equals(rule.type, CSSRule.KEYFRAME_RULE, "Should be a keyframe rule");
+  assert_equals(rule.style.length, 1, "animation-name doesn't apply to keyframes");
+  assert_equals(rule.style.marginTop, "10px", "Should have a margin-top declaration");
+  rule.style.setProperty("animation-name", "none");
+  assert_equals(rule.style.getPropertyValue("animation-name"), "", "Shouldn't have been able to set the animation-nameproperty via setProperty");
+  assert_equals(rule.style.length, 1, "Shouldn't have been able to set animation-name via setProperty");
+  rule.style.cssText = "margin-bottom: 10px; animation-name: none;";
+  assert_equals(rule.style.length, 1, "Should not have been able to set animation-name via cssText");
+  assert_equals(rule.style.marginBottom, "10px", "Should have a margin-bottom declaration");
+
+}, "@keyframe");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/selectorSerialize.html b/testing/web-platform/tests/css/cssom/selectorSerialize.html
new file mode 100644
index 0000000000..f3a402f5f0
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/selectorSerialize.html
@@ -0,0 +1,121 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM Test: test serialized selector which is only one simple selector in the sequence of simple selectors</title>
+        <link rel="author" title="T.Nishitani" href="mailto:lequinharay@gmail.com">
+        <link rel="reviewer" title="L. David Baron" href="https://dbaron.org/">
+        <link rel="help" href="https://drafts.csswg.org/cssom-1/#serializing-selectors">
+        <meta name="flags" content="dom">
+        <meta charset="utf-8">
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+        <style id="teststyles">
+        </style>
+    </head>
+    <body>
+        <div id="log"></div>
+        <script>
+            function assert_selector_serializes_to(source, expected_result) {
+              var style_element = document.getElementById("teststyles");
+              style_element.firstChild.data = source + "{ font-size: 1em; }";
+              var sheet = style_element.sheet;
+              assert_equals(sheet.cssRules[sheet.cssRules.length - 1].selectorText, expected_result);
+            }
+
+            function run_tests_on_anplusb_selector(source) {
+                assert_selector_serializes_to(source + '( even   )', source + '(2n)');
+                assert_selector_serializes_to(source + '(   odd )', source + '(2n+1)');
+                assert_selector_serializes_to(source + '( +10  )', source + '(10)');
+                assert_selector_serializes_to(source + '(   -10 )', source + '(-10)');
+                assert_selector_serializes_to(source + '( +4n  )', source + '(4n)');
+                assert_selector_serializes_to(source + '( -3n   )', source + '(-3n)');
+                assert_selector_serializes_to(source + '( 1n + 5  )', source + '(n+5)');
+                assert_selector_serializes_to(source + '( -1n +     5 )', source + '(-n+5)');
+                assert_selector_serializes_to(source + '( -1n     - 5 )', source + '(-n-5)');
+            }
+
+            test(function() {
+              assert_selector_serializes_to(":nth-child(  3n - 0)", ":nth-child(3n)");
+              assert_selector_serializes_to(":nth-child(  1n - 0)", ":nth-child(n)");
+            }, ":nth-child serialization produces canonical form");
+
+
+            /* for universal selecter with default namespace */
+            test(function(){
+                /* this is single universal selector */
+                assert_selector_serializes_to('*', '*');
+                assert_selector_serializes_to(' * ', '*');
+            }, 'single universal selector shows \'*\' when serialized.')
+
+            test(function(){
+                assert_selector_serializes_to('div', 'div');
+                assert_selector_serializes_to(' div ', 'div');
+            }, 'single type (simple) selector in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                assert_selector_serializes_to('.class', '.class');
+                assert_selector_serializes_to(' .class    ', '.class');
+            }, 'single class (simple) selector in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                assert_selector_serializes_to('#id', '#id');
+                assert_selector_serializes_to('     #id ', '#id');
+            }, 'single id (simple) selector in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                assert_selector_serializes_to(':hover', ':hover');
+                assert_selector_serializes_to('    :hover    ', ':hover');
+            }, 'single pseudo (simple) selector which does not accept arguments in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                assert_selector_serializes_to(':lang(ja)', ':lang(ja)');
+                assert_selector_serializes_to(':lang( ja )', ':lang(ja)');
+                assert_selector_serializes_to(':lang( j\\ a )', ':lang(j\\ a)');
+            }, 'single pseudo (simple) selector "lang" which accepts arguments in the sequence of simple selectors that is not a universal selector')
+
+
+            test(function(){
+                run_tests_on_anplusb_selector(':nth-child');
+            }, 'single pseudo (simple) selector "nth-child" which accepts arguments in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                run_tests_on_anplusb_selector(':nth-last-child');
+            }, 'single pseudo (simple) selector "nth-last-child" which accepts arguments in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                run_tests_on_anplusb_selector(':nth-of-type');
+            }, 'single pseudo (simple) selector "nth-of-child" which accepts arguments in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                run_tests_on_anplusb_selector(':nth-last-of-type');
+            }, 'single pseudo (simple) selector ":nth-last-of-type" which accepts arguments in the sequence of simple selectors that is not a universal selector')
+
+            test(function(){
+                assert_selector_serializes_to(' :not( abc ) ', ':not(abc)');
+                assert_selector_serializes_to(' :not(  .head ) ', ':not(.head)');
+                assert_selector_serializes_to(' :not(  #head   ) ', ':not(#head)');
+                assert_selector_serializes_to(' :not(  :hover   ) ', ':not(:hover)');
+            }, 'single pseudo (simple) selector ":not" which accepts arguments in the sequence of simple selectors that is not a universal selector')
+
+            var escaped_ns_rule = "@namespace ns\\:odd url(ns);";
+            test(function() {
+                assert_selector_serializes_to("[ns\\:foo]", "[ns\\:foo]");
+            }, "escaped character in attribute name");
+            test(function() {
+                assert_selector_serializes_to("[\\30zonk]", "[\\30 zonk]");
+            }, "escaped character as code point in attribute name");
+            test(function() {
+                assert_selector_serializes_to("[\\@]", "[\\@]");
+            }, "escaped character (@) in attribute name");
+            test(function() {
+                assert_selector_serializes_to("[*|ns\\:foo]", "[*|ns\\:foo]");
+            }, "escaped character in attribute name with any namespace");
+            test(function() {
+                assert_selector_serializes_to(escaped_ns_rule + "[ns\\:odd|foo]", "[ns\\:odd|foo]");
+            }, "escaped character in attribute prefix");
+            test(function() {
+                assert_selector_serializes_to(escaped_ns_rule + "[ns\\:odd|odd\\:name]", "[ns\\:odd|odd\\:name]");
+            }, "escaped character in both attribute prefix and name");
+        </script>
+    </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-001-ref.html b/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-001-ref.html
new file mode 100644
index 0000000000..74e5807470
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-001-ref.html
@@ -0,0 +1,13 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>(Ref #1) CSSOM - CSSStyleRule.selectorText Modification Restyle - Reference #1</title>
+
+<style>
+div {
+  color: green;
+}
+</style>
+
+<body>
+<div>I should be green.</div>
+</body>
diff --git a/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-001.html b/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-001.html
new file mode 100644
index 0000000000..43a76d1ef2
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-001.html
@@ -0,0 +1,22 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>(Test #1) CSSOM - CSSStyleRule.selectorText Modification Restyle - Test #1</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstylerule-selectortext">
+<link rel="match" href="selectorText-modification-restyle-001-ref.html">
+
+<style>
+@namespace bogus url(http://example.com/bogus);
+
+bogus|div {
+  color: green;
+}
+</style>
+
+<body>
+<div>I should be green.</div>
+<script>
+// Remove the "bogus" namespace--now it should apply to the div above.
+// We also expect to see a restyle.
+document.querySelector("style").sheet.cssRules[1].selectorText = "div";
+</script>
+</body>
diff --git a/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-002.html b/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-002.html
new file mode 100644
index 0000000000..a6b37c2cf0
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/selectorText-modification-restyle-002.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<title>CSSOM: Modify selectorText in a shadow tree stylesheet</title>
+<link rel="author" title="Rune Lillesveen" href="mailto:futhark@chromium.org">
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstylerule-selectortext">
+<link rel="help" href="https://drafts.csswg.org/css-scoping/#selectors">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  #container { color: red }
+  .subtree * { color: pink }
+</style>
+<div id="container">
+  <div id="host"></div>
+</div>
+<script>
+  const root = host.attachShadow({mode:"open"});
+  root.innerHTML = "<style>nomatch { color: green }</style><div>Green</div>";
+  const div = root.querySelector("div");
+
+  test(() => {
+    assert_equals(getComputedStyle(div).color, "rgb(255, 0, 0)", "Color should initial be red.");
+  }, "Check initial color.");
+
+  test(() => {
+    // The combination of the .subtree and CSSOM selector style invalidations
+    // caused the Blink implementation to fail an assertion.
+    container.className = "subtree";
+    root.styleSheets[0].cssRules[0].selectorText = "div";
+    assert_equals(getComputedStyle(div).color, "rgb(0, 128, 0)", "Color should be green after stylesheet change.");
+  }, "Check that color changes correctly after shadow stylesheet selector and #container class is changed.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/serialization-CSSDeclaration-with-important.html b/testing/web-platform/tests/css/cssom/serialization-CSSDeclaration-with-important.html
new file mode 100644
index 0000000000..804885a74a
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/serialization-CSSDeclaration-with-important.html
@@ -0,0 +1,32 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>cssom - Serialization of CSS declaration with "important" flag</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-declaration">
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<div id="noWhitespace" style="display: inline !important;"></div>
+<div id="whitespace" style="background-color: blue !important; color: red ! important;"></div>
+<div id="dinamicallyStyle"></div>
+<script>
+    test(function () {
+        var css_style = document.querySelector('#noWhitespace').style.cssText;
+        assert_equals(css_style, "display: inline !important;");
+    }, "Inline style declaration without whitespace between '!' and 'important'.");
+
+    test(function () {
+        var css_style = document.querySelector('#whitespace').style.cssText;
+        assert_equals(css_style, "background-color: blue !important; color: red !important;");
+    }, "Inline style declaration with whitespace between '!' and 'important'.");
+
+    test(function () {
+        document.querySelector('#dinamicallyStyle').style.cssText = "color: black !important;";
+        var css_style = document.querySelector('#dinamicallyStyle').style.cssText;
+        assert_equals(css_style, "color: black !important;");
+    }, "Style set dynamically via cssText without whitespace between '!' and 'important'.");
+
+    test(function () {
+        document.querySelector('#dinamicallyStyle').style.cssText = "color: black ! important;";
+        var css_style = document.querySelector('#dinamicallyStyle').style.cssText;
+        assert_equals(css_style, "color: black !important;");
+    }, "Style set dynamically via cssText with whitespace between '!' and 'important'.");
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/serialize-all-longhands.html b/testing/web-platform/tests/css/cssom/serialize-all-longhands.html
new file mode 100644
index 0000000000..95204b6361
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/serialize-all-longhands.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Serialize all longhands</title>
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com" />
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-getpropertyvalue">
+<meta name="assert" content="Checks that all longhands indexed in a CSSStyleDeclaration can be serialized to a non-empty string when set to their initial value.">
+
+<div id="target"></div>
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+function nonSerializableProperties(style) {
+  const result = [];
+  assert_greater_than(style.length, 0, "Should have longhands");
+  for (let property of style) {
+    if (!style.getPropertyValue(property)) {
+      result.push(property);
+    }
+  }
+  return result;
+}
+
+const target = document.getElementById("target");
+target.style.cssText = "all: initial; direction: initial; unicode-bidi: initial;";
+
+test(function() {
+  const props = nonSerializableProperties(target.style);
+  assert_array_equals(props, []);
+}, "Specified style");
+
+test(function() {
+  const props = nonSerializableProperties(getComputedStyle(target));
+  assert_array_equals(props, []);
+}, "Computed style");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/serialize-custom-props.html b/testing/web-platform/tests/css/cssom/serialize-custom-props.html
new file mode 100644
index 0000000000..cfe96ff0aa
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/serialize-custom-props.html
@@ -0,0 +1,69 @@
+<link rel=author title="Tab Atkins-Bittner" href="https://www.xanthir.com/contact/">
+<link rel="help" href="https://drafts.csswg.org/css-values/#calc-range">
+<body><!doctype html>
+<title>Serializing Integers Never Uses Scinot</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<link rel=author title="Tab Atkins-Bittner" href="https://www.xanthir.com/contact/">
+<link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-component-value">
+<!--
+  Per CSSOM, integers always serialize all their digits out.
+  They never serialize to scinot, regardless of size,
+  because that makes them stop being an integer.
+  This applies to custom properties as well.
+-->
+<body>
+
+<script>
+
+try {
+CSS.registerProperty({
+    name: "--two",
+    value: "<integer>",
+    inherits: true,
+    initial: -1
+});
+}catch(e){}
+
+testIntLength(4);
+testIntLength(6);
+testIntLength(8);
+testIntLength(12);
+// JS starts serializing with scinot at 22 digits...
+testIntLength(25);
+
+function testIntLength(len) {
+    let el = document.body;
+    const val = "1".repeat(len);
+    test(()=>{
+        el.removeAttribute("style");
+        const nullVal = getComputedStyle(el).zIndex;
+        el.style.zIndex=val;
+        assert_not_equals(getComputedStyle(el).zIndex, nullVal)
+    }, `z-index can take a ${len}-digit integer`);
+
+    test(()=>{
+        el.removeAttribute("style");
+        el.style.setProperty("--one", val);
+        assert_equals(getComputedStyle(el).getPropertyValue("--one"), val);
+    }, `An unregistered custom prop can take a ${len}-digit integer`);
+
+    test(()=>{
+        el.removeAttribute("style");
+        el.style.setProperty("--two", val);
+        assert_equals(getComputedStyle(el).getPropertyValue("--two"), val);
+    }, `An <integer> custom prop can take a ${len}-digit integer`);
+
+    test(()=>{
+        el.removeAttribute("style");
+        el.style.zIndex = val;
+        const standardVal = getComputedStyle(el).zIndex;
+        el.removeAttribute("style");
+        el.style.setProperty("--three", val);
+        el.style.zIndex = "var(--three)";
+        assert_equals(getComputedStyle(el).zIndex, standardVal);
+    }, `z-index can take a custom property set to a ${len}-digit integer`);
+}
+
+</script>
diff --git a/testing/web-platform/tests/css/cssom/serialize-media-rule.html b/testing/web-platform/tests/css/cssom/serialize-media-rule.html
new file mode 100644
index 0000000000..90561fdf70
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/serialize-media-rule.html
@@ -0,0 +1,185 @@
+<!doctype html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSSOM - Serialization of CSSMediaRule</title>
+  <link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-rule">
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script>
+function makeSheet(t) {
+  var style = document.createElement('style');
+  document.body.appendChild(style);
+  t.add_cleanup(function() {
+    document.body.removeChild(style);
+  });
+  return style.sheet;
+}
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media {}', 0);
+
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(sheet.cssRules[0].cssText, '@media  {\n}');
+}, 'empty media query list');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media all {}');
+  sheet.insertRule('@media print {}');
+  sheet.insertRule('@media screen {}');
+  sheet.insertRule('@media speech {}');
+
+  assert_equals(sheet.cssRules.length, 4);
+  assert_equals(sheet.cssRules[0].cssText, '@media speech {\n}');
+  assert_equals(sheet.cssRules[1].cssText, '@media screen {\n}');
+  assert_equals(sheet.cssRules[2].cssText, '@media print {\n}');
+  assert_equals(sheet.cssRules[3].cssText, '@media all {\n}');
+}, 'type - no features');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media not all {}');
+  sheet.insertRule('@media not print {}');
+  sheet.insertRule('@media not screen {}');
+  sheet.insertRule('@media not speech {}');
+
+  assert_equals(sheet.cssRules.length, 4);
+  assert_equals(sheet.cssRules[0].cssText, '@media not speech {\n}');
+  assert_equals(sheet.cssRules[1].cssText, '@media not screen {\n}');
+  assert_equals(sheet.cssRules[2].cssText, '@media not print {\n}');
+  assert_equals(sheet.cssRules[3].cssText, '@media not all {\n}');
+}, 'type - no features - negation');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media aLL {}');
+  sheet.insertRule('@media pRiNt {}');
+  sheet.insertRule('@media screEN {}');
+  sheet.insertRule('@media spEech {}');
+
+  assert_equals(sheet.cssRules.length, 4);
+  assert_equals(sheet.cssRules[0].cssText, '@media speech {\n}');
+  assert_equals(sheet.cssRules[1].cssText, '@media screen {\n}');
+  assert_equals(sheet.cssRules[2].cssText, '@media print {\n}');
+  assert_equals(sheet.cssRules[3].cssText, '@media all {\n}');
+}, 'type - no features - character case normalization');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media all and (color) {}');
+
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(sheet.cssRules[0].cssText, '@media (color) {\n}');
+}, 'type - omission of all');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media not all and (color) {}');
+
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(sheet.cssRules[0].cssText, '@media not all and (color) {\n}');
+}, 'type - inclusion of negated all');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media screen and (Color) {}');
+  sheet.insertRule('@media screen and (cOLor) {}');
+
+  assert_equals(sheet.cssRules.length, 2);
+  assert_equals(sheet.cssRules[0].cssText, '@media screen and (color) {\n}');
+  assert_equals(sheet.cssRules[1].cssText, '@media screen and (color) {\n}');
+}, 'features - character case normalization');
+
+/**
+ * The following test is disabled pending clarification of the intended
+ * behavior: https://github.com/w3c/csswg-drafts/issues/533
+ */
+//test(function(t) {
+//  var sheet = makeSheet(t);
+//  sheet.insertRule('@media screen and (color) and (color) {}');
+//
+//  assert_equals(sheet.cssRules.length, 1);
+//  assert_equals(
+//    sheet.cssRules[0].cssText,
+//    '@media screen and (color) {\n}'
+//  );
+//}, 'features - de-duplication');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media print and (max-width: 23px) and (max-width: 45px) {}');
+
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(
+    sheet.cssRules[0].cssText,
+    '@media print and (max-width: 23px) and (max-width: 45px) {\n}'
+  );
+}, 'features - preservation of overspecified features');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media screen and (max-width: 0px) and (color) {}');
+  sheet.insertRule('@media screen and (color) and (max-width: 0px) {}');
+
+  assert_equals(sheet.cssRules.length, 2);
+  assert_equals(
+    sheet.cssRules[0].cssText,
+    '@media screen and (color) and (max-width: 0px) {\n}'
+  );
+  assert_equals(
+    sheet.cssRules[1].cssText,
+    '@media screen and (max-width: 0px) and (color) {\n}'
+  );
+}, 'features - no lexicographical sorting');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media screen and (max-width: 0px), screen and (color) {}');
+
+  assert_equals(sheet.cssRules.length, 1);
+  assert_equals(
+    sheet.cssRules[0].cssText,
+    '@media screen and (max-width: 0px), screen and (color) {\n}'
+  );
+}, 'media query list');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media print {}');
+
+  assert_equals(sheet.cssRules.length, 1);
+  sheet.cssRules[0].insertRule('#foo { z-index: 23; float: left; }', 0);
+  assert_equals(
+    sheet.cssRules[0].cssText,
+    [
+      '@media print {',
+      '  #foo { z-index: 23; float: left; }',
+      '}'
+    ].join('\n')
+  );
+}, 'one rule');
+
+test(function(t) {
+  var sheet = makeSheet(t);
+  sheet.insertRule('@media print {}');
+
+  assert_equals(sheet.cssRules.length, 1);
+  sheet.cssRules[0].insertRule('#foo { z-index: 23; float: left; }', 0);
+  sheet.cssRules[0].insertRule('#bar { float: none; z-index: 45; }', 0);
+  assert_equals(
+    sheet.cssRules[0].cssText,
+    [
+      '@media print {',
+      '  #bar { float: none; z-index: 45; }',
+      '  #foo { z-index: 23; float: left; }',
+      '}'
+    ].join('\n')
+  );
+}, 'many rules');
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/serialize-namespaced-type-selectors.html b/testing/web-platform/tests/css/cssom/serialize-namespaced-type-selectors.html
new file mode 100644
index 0000000000..600008c7a9
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/serialize-namespaced-type-selectors.html
@@ -0,0 +1,257 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>CSSOM Test: test serialization of type selectors and namespace prefixes</title>
+        <link rel="author" title="Rune Lillesveen" href="mailto:rune@opera.com">
+        <link rel="help" href="https://drafts.csswg.org/cssom-1/#serializing-selectors">
+        <meta name="flags" content="dom">
+        <meta charset="utf-8">
+        <script src="/resources/testharness.js"></script>
+        <script src="/resources/testharnessreport.js"></script>
+        <style id="teststyles">
+        </style>
+    </head>
+    <body>
+        <div id="log"></div>
+        <script>
+            var ns_rule = "@namespace ns url(ns);";
+            var default_ns_rules = "@namespace url(default_ns); @namespace nsdefault url(default_ns);" + ns_rule;
+
+            function assert_selector_serializes_to(source, expected_result) {
+                var style_element = document.getElementById("teststyles");
+                style_element.firstChild.data = source + "{ font-size: 1em; }";
+                var sheet = style_element.sheet;
+                assert_equals(sheet.cssRules[sheet.cssRules.length - 1].selectorText, expected_result);
+            }
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "e", "e");
+                assert_selector_serializes_to(default_ns_rules + "e", "e");
+            }, "Simple type selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|e", "|e");
+                assert_selector_serializes_to(default_ns_rules + "|e", "|e");
+            }, "Type selector without a namespace");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|e", "e");
+                assert_selector_serializes_to(default_ns_rules + "*|e", "*|e");
+            }, "Type selector with any namespace");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*", "*");
+                assert_selector_serializes_to(default_ns_rules + "*", "*");
+            }, "Universal selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|*", "|*");
+                assert_selector_serializes_to(default_ns_rules + "|*", "|*");
+            }, "Universal selector without a namespace");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|*", "*");
+                assert_selector_serializes_to(default_ns_rules + "*|*", "*|*");
+            }, "Universal selector in any namespace");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|e", "ns|e");
+                assert_selector_serializes_to(default_ns_rules + "ns|e", "ns|e");
+            }, "Type selector with namespace");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|*", "ns|*");
+                assert_selector_serializes_to(default_ns_rules + "ns|*", "ns|*");
+            }, "Universal selector with namespace");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "e.c", "e.c");
+                assert_selector_serializes_to(default_ns_rules + "e.c", "e.c");
+            }, "Simple type selector followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "e#i", "e#i");
+                assert_selector_serializes_to(default_ns_rules + "e#i", "e#i");
+            }, "Simple type selector followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "e:hover", "e:hover");
+                assert_selector_serializes_to(default_ns_rules + "e:hover", "e:hover");
+            }, "Simple type selector followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "e::before", "e::before");
+                assert_selector_serializes_to(default_ns_rules + "e::before", "e::before");
+            }, "Simple type selector followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "e[attr]", "e[attr]");
+                assert_selector_serializes_to(default_ns_rules + "e[attr]", "e[attr]");
+            }, "Simple type selector followed by atttribute selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|e.c", "|e.c");
+                assert_selector_serializes_to(default_ns_rules + "|e.c", "|e.c");
+            }, "Type selector without a namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|e#i", "|e#i");
+                assert_selector_serializes_to(default_ns_rules + "|e#i", "|e#i");
+            }, "Type selector without a namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|e:hover", "|e:hover");
+                assert_selector_serializes_to(default_ns_rules + "|e:hover", "|e:hover");
+            }, "Type selector without a namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|e::before", "|e::before");
+                assert_selector_serializes_to(default_ns_rules + "|e::before", "|e::before");
+            }, "Type selector without a namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|e[attr]", "|e[attr]");
+                assert_selector_serializes_to(default_ns_rules + "|e[attr]", "|e[attr]");
+            }, "Type selector without a namespace followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|e.c", "e.c");
+                assert_selector_serializes_to(default_ns_rules + "*|e.c", "*|e.c");
+            }, "Type selector with any namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|e#id", "e#id");
+                assert_selector_serializes_to(default_ns_rules + "*|e#id", "*|e#id");
+            }, "Type selector with any namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|e:hover", "e:hover");
+                assert_selector_serializes_to(default_ns_rules + "*|e:hover", "*|e:hover");
+            }, "Type selector with any namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|e::before", "e::before");
+                assert_selector_serializes_to(default_ns_rules + "*|e::before", "*|e::before");
+            }, "Type selector with any namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|e[attr]", "e[attr]");
+                assert_selector_serializes_to(default_ns_rules + "*|e[attr]", "*|e[attr]");
+            }, "Type selector with any namespace followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*.c", ".c");
+                assert_selector_serializes_to(default_ns_rules + "*.c", ".c");
+            }, "Universal selector followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*#i", "#i");
+                assert_selector_serializes_to(default_ns_rules + "*#i", "#i");
+            }, "Universal selector followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*:hover", ":hover");
+                assert_selector_serializes_to(default_ns_rules + "*:hover", ":hover");
+            }, "Universal selector followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*::before", "::before");
+                assert_selector_serializes_to(default_ns_rules + "*::before", "::before");
+            }, "Universal selector followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*[attr]", "[attr]");
+                assert_selector_serializes_to(default_ns_rules + "*[attr]", "[attr]");
+            }, "Universal selector followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|*.c", "|*.c");
+                assert_selector_serializes_to(default_ns_rules + "|*.c", "|*.c");
+            }, "Universal selector without a namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|*#i", "|*#i");
+                assert_selector_serializes_to(default_ns_rules + "|*#i", "|*#i");
+            }, "Universal selector without a namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|*:hover", "|*:hover");
+                assert_selector_serializes_to(default_ns_rules + "|*:hover", "|*:hover");
+            }, "Universal selector without a namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|*::before", "|*::before");
+                assert_selector_serializes_to(default_ns_rules + "|*::before", "|*::before");
+            }, "Universal selector without a namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "|*[attr]", "|*[attr]");
+                assert_selector_serializes_to(default_ns_rules + "|*[attr]", "|*[attr]");
+            }, "Universal selector without a namespace followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|*.c", ".c");
+                assert_selector_serializes_to(default_ns_rules + "*|*.c", "*|*.c");
+            }, "Universal selector in any namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|*#id", "#id");
+                assert_selector_serializes_to(default_ns_rules + "*|*#id", "*|*#id");
+            }, "Universal selector in any namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|*:hover", ":hover");
+                assert_selector_serializes_to(default_ns_rules + "*|*:hover", "*|*:hover");
+            }, "Universal selector in any namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|*::before", "::before");
+                assert_selector_serializes_to(default_ns_rules + "*|*::before", "*|*::before");
+            }, "Universal selector in any namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "*|*[attr]", "[attr]");
+                assert_selector_serializes_to(default_ns_rules + "*|*[attr]", "*|*[attr]");
+            }, "Universal selector in any namespace followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|e.c", "ns|e.c");
+                assert_selector_serializes_to(default_ns_rules + "ns|e.c", "ns|e.c");
+            }, "Type selector with namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|e#i", "ns|e#i");
+                assert_selector_serializes_to(default_ns_rules + "ns|e#i", "ns|e#i");
+            }, "Type selector with namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|e:hover", "ns|e:hover");
+                assert_selector_serializes_to(default_ns_rules + "ns|e:hover", "ns|e:hover");
+            }, "Type selector with namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|e::before", "ns|e::before");
+                assert_selector_serializes_to(default_ns_rules + "ns|e::before", "ns|e::before");
+            }, "Type selector with namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|e[attr]", "ns|e[attr]");
+                assert_selector_serializes_to(default_ns_rules + "ns|e[attr]", "ns|e[attr]");
+            }, "Type selector with namespace followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|*.c", "ns|*.c");
+                assert_selector_serializes_to(default_ns_rules + "ns|*.c", "ns|*.c");
+            }, "Universal selector with namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|*#i", "ns|*#i");
+                assert_selector_serializes_to(default_ns_rules + "ns|*#i", "ns|*#i");
+            }, "Universal selector with namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|*:hover", "ns|*:hover");
+                assert_selector_serializes_to(default_ns_rules + "ns|*:hover", "ns|*:hover");
+            }, "Universal selector with namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|*::before", "ns|*::before");
+                assert_selector_serializes_to(default_ns_rules + "ns|*::before", "ns|*::before");
+            }, "Universal selector with namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(ns_rule + "ns|*[attr]", "ns|*[attr]");
+                assert_selector_serializes_to(default_ns_rules + "ns|*[attr]", "ns|*[attr]");
+            }, "Universal selector with namespace followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|e", "e");
+            }, "Type selector with namespace equal to default namespace");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|*", "*");
+            }, "Universal selector with namespace equal to default namespace");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|e.c", "e.c");
+            }, "Type selector with namespace equal to default namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|e#i", "e#i");
+            }, "Type selector with namespace equal to default namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|e:hover", "e:hover");
+            }, "Type selector with namespace equal to default namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|e::before", "e::before");
+            }, "Type selector with namespace equal to default namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|e[attr]", "e[attr]");
+            }, "Type selector with namespace equal to default namespace followed by attribute selector");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|*.c", ".c");
+            }, "Universal selector with namespace equal to default namespace followed by class");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|*#i", "#i");
+            }, "Universal selector with namespace equal to default namespace followed by id");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|*:hover", ":hover");
+            }, "Universal selector with namespace equal to default namespace followed by pseudo class");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|*::before", "::before");
+            }, "Universal selector with namespace equal to default namespace followed by pseudo element");
+            test(function() {
+                assert_selector_serializes_to(default_ns_rules + "nsdefault|*[attr]", "[attr]");
+            }, "Universal selector with namespace equal to default namespace followed by attribute selector");
+        </script>
+    </body>
+</html>
+
diff --git a/testing/web-platform/tests/css/cssom/serialize-values.html b/testing/web-platform/tests/css/cssom/serialize-values.html
new file mode 100644
index 0000000000..e92808af70
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/serialize-values.html
@@ -0,0 +1,620 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSSOM serialize values</title>
+<link rel="help" href="https://drafts.csswg.org/cssom/#serializing-css-values">
+<meta name="author" title="Josh Matthews" href="mailto:josh@joshmatthews.net">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body>
+  <div id="log"></div>
+  <div id="parent"></div>
+  <script>
+    function iterable(values) {
+      var i = 0;
+      return function() {
+        if (i < values.length) {
+          return values[i++];
+        }
+        return null;
+      }
+    }
+
+    function color() {
+      var colors = ['black', 'red', 'rgb(50, 75, 100)', 'rgba(5, 7, 10, 0.5)'];
+      return iterable(colors);
+    }
+
+    function percentage() {
+      var values = ["5%", {actual: ".5%", serialized: "0.5%"}];
+      return iterable(values);
+    }
+
+    function negative_percentage() {
+      var values = ["-5%", {actual: "-.5%", serialized: "-0.5%"}];
+      return iterable(values);
+    }
+
+    function length() {
+      var values = ["0px", "1px", {actual: ".1em", serialized: "0.1em"}];
+      return iterable(values);
+    }
+
+    function negative_length() {
+      var values = [{actual: "-0px", serialized: "0px"},
+                    "-1px", {actual: "-.1em", serialized: "-0.1em"}];
+      return iterable(values);
+    }
+
+    function degree() {
+      var values = ["87deg"];
+      return iterable(values);
+    }
+
+    function uri() {
+      var values = ["url(\"http://localhost/\")",
+        {actual: "url(http://localhost/)",
+         serialized: "url(\"http://localhost/\")"}];
+      return iterable(values);
+    }
+
+    function border_style() {
+      var values = ['none', 'hidden', 'dotted', 'dashed', 'solid', 'double', 'groove', 'ridge',
+                    'inset', 'outset'];
+      return iterable(values);
+    }
+
+    function border_style_without_hidden() {
+      var values = ['none', 'dotted', 'dashed', 'solid', 'double', 'groove', 'ridge',
+                    'inset', 'outset'];
+      return iterable(values);
+    }
+
+    function integer() {
+      var values = ['0', '101', '-51'];
+      return iterable(values);
+    }
+
+    function nonzero_positive_integer() {
+      var values = ['101'];
+      return iterable(values);
+    }
+
+    function shape() {
+      var values = ['rect(1em, auto, 0.5px, 2000em)'];
+      return iterable(values);
+    }
+
+    function string() {
+      var values = ['"string"', {actual: "'string'", serialized: '"string"'}];
+      return iterable(values);
+    }
+
+    function counter() {
+      var values = ['counter(par-num)',
+                    { actual: 'counter(par-num, decimal)', serialized: 'counter(par-num)' },
+                    'counter(par-num, upper-roman)'];
+      return iterable(values);
+    }
+
+    function attr() {
+      var values = ['attr(foo-bar)', 'attr(foo_bar)'];
+      return iterable(values);
+    }
+
+    function family_name() {
+      var values = ['Arial', {actual: "'Lucida Grande'", serialized: '"Lucida Grande"'}];
+      return iterable(values);
+    }
+
+    function generic_family() {
+      var values = ['serif', 'sans-serif'];
+      return iterable(values);
+    }
+
+    function absolute_size() {
+      var values = ['xx-small', 'x-small', 'small', 'medium', 'large', 'x-large', 'xx-large'];
+      return iterable(values);
+    }
+
+    function relative_size() {
+      var values = ['larger', 'smaller'];
+      return iterable(values);
+    }
+
+    function number() {
+      var values = ['0', {'actual': '-0', serialized: '0'}, '1000', '-5123', '0.9', '-0.09'];
+      return iterable(values);
+    }
+
+    function positive_number() {
+      var values = ['0', {'actual': '-0', serialized: '0'}, '1000', '0.9'];
+      return iterable(values);
+    }
+
+    function generate_inline_style(name, value) {
+      if (value) {
+        return {'declaration': name + ": " + value.actual,
+                'value': value.actual,
+                'result': value.expected};
+      }
+      return null;
+    }
+
+    // A more elegant way of doing this would be better, but for now this exception list handles cases where "center" must be omitted.
+    var minimal_results = {
+        "background-position: 5% center": "5%",
+        "background-position: 0.5% center": "0.5%",
+        "background-position: -5% center": "-5%",
+        "background-position: -0.5% center": "-0.5%",
+        "background-position: 0px center": "0px",
+        "background-position: 1px center": "1px",
+        "background-position: 0.1em center": "0.1em",
+        "background-position: 0px center": "0px",
+        "background-position: -1px center": "-1px",
+        "background-position: -0.1em center": "-0.1em",
+        "background-position: left center": "left",
+        "background-position: center top": "top",
+        "background-position: center center": "center",
+        "background-position: center bottom": "bottom",
+        "background-position: right center": "right",
+    };
+
+    function create_result(propertyName, actual, expected) {
+      var key = propertyName + ": " + expected
+      if (key in minimal_results)
+        expected = minimal_results[key]
+      return {actual: actual, expected: expected}
+    }
+
+    function all_values(propertyName, values) {
+      var results = [];
+      for (var i = 0; i < values.length; i++) {
+        var value = values[i];
+        if (typeof value == "function") {
+          var f = value();
+          var result;
+          while ((result = f()) != null) {
+            if (typeof result == "object" && 'serialized' in result) {
+              results.push(create_result(propertyName, result.actual, result.serialized));
+            } else {
+              results.push(create_result(propertyName, result, result));
+            }
+          }
+        } else if (typeof value == "string") {
+          results.push(create_result(propertyName, value, value));
+        } else if (value instanceof Array) {
+          var subresults = [];
+          for (var j = 0; j < value.length; j++) {
+            var subresult = all_values(propertyName, value[j]);
+            if (!(subresult instanceof Array)) {
+              subresult = [subresult];
+            }
+            subresults.push(subresult);
+          }
+          if (subresults.length > 1) {
+            function choose_slices(vecs) {
+              if (vecs.length == 1) {
+                return vecs[0].map(function(v) { return [v]; });
+              }
+              var slice_results = [];
+              var rest = choose_slices(vecs.slice(1, vecs.length));
+              for (var a = 0; a < vecs[0].length; a++) {
+                for (var b = 0; b < rest.length; b++) {
+                  var result = vecs[0][a];
+                  slice_results.push([result].concat(rest[b]));
+                }
+              }
+              return slice_results;
+            }
+
+            subresults = choose_slices(subresults).map(function (a) {
+              var actual = a.map(function(a) { return a.actual });
+              var expected = a.map(function(a) { return a.expected });
+              return create_result(propertyName, actual.join(' '), expected.join(' '))
+            });
+          }
+          for (var j = 0; j < subresults.length; j++) {
+            results = results.concat(subresults[j]);
+          }
+        } else if (value instanceof Object && 'serialized' in value) {
+          results.push(create_result(propertyName, value.actual, value.serialized));
+        } else if (typeof value == "number") {
+          results.push(create_result(propertyName, value.toString(), value.toString()));
+        } else {
+          throw "unexpected value type: " + typeof(value);
+        }
+      }
+      return results;
+    }
+
+    function create_value_generator(propertyName, property) {
+      var results = all_values(propertyName, property.values);
+      return iterable(results);
+    }
+
+    function to_idl(property) {
+      return property.replace(/-\w/g, function(x){return x[1].toUpperCase()});
+    }
+
+    function run_individual_test(property, generator, initial) {
+      var elem = document.createElement('div');
+      document.getElementById('parent').appendChild(elem);
+      var test_data = generator();
+      var style = generate_inline_style(property, test_data);
+      if (!style) {
+        return false;
+      }
+      var t = async_test(style.declaration);
+
+      t.add_cleanup(function() {
+        document.getElementById('parent').removeChild(elem);
+      });
+
+      t.step(function() {
+        elem.setAttribute('style', style.declaration);
+        var expected = style.result;
+        var serialized = elem.style[to_idl(property)];
+        assert_equals(serialized, expected, property + ' raw inline style declaration');
+        elem.setAttribute('style', '');
+        elem.style[to_idl(property)] = style.value;
+        assert_equals(elem.style[to_idl(property)], expected, property + ' style property');
+      });
+      t.done();
+      return true;
+    }
+
+    function test_property(property) {
+      var generator = create_value_generator(property[0], property[1]);
+      while (run_individual_test(property[0], generator, property[1].initial)) {
+      }
+    }
+
+    var properties = [
+      ['background-attachment', {
+        'values': ['scroll', 'fixed', 'inherit'],
+        'initial': 'scroll',
+      }],
+      ['background-color', {
+        'values': [color, 'transparent', 'inherit'],
+        'initial': 'transparent',
+      }],
+      ['background-image', {
+        'values': [uri, 'none', 'inherit'],
+        'initial': 'none',
+      }],
+      ['background-position', {
+        'values': [[[percentage, negative_percentage, length, negative_length,
+                     'left', 'center', 'right'],
+                    [percentage, negative_percentage, length, negative_length,
+                     'top', 'center', 'bottom']],
+                   'inherit'],
+        'initial': '0% 0%',
+      }],
+      ['background-repeat', {
+        'values': ['repeat', 'repeat-x', 'repeat-y', 'no-repeat', 'inherit'],
+        'initial': 'repeat',
+      }],
+      //background
+      ['border-collapse', {
+        'values': ['collapse', 'separate', 'inherit'],
+        'initial': 'separate',
+      }],
+      //border-color
+      ['border-spacing', {
+        'values': [length, 'inherit'],
+        'initial': '0',
+      }],
+      //border-style
+      //border-top, border-right, border-bottom, border-left
+      ['border-top-color', {
+        'values': [color, 'transparent', 'inherit'],
+        'initial': 'black', //FIXME
+      }],
+      ['border-right-color', {
+        'values': [color, 'transparent', 'inherit'],
+        'initial': 'black', //FIXME
+      }],
+      ['border-bottom-color', {
+        'values': [color, 'transparent', 'inherit'],
+        'initial': 'black', //FIXME
+      }],
+      ['border-left-color', {
+        'values': [color, 'transparent', 'inherit'],
+        'initial': 'black', //FIXME
+      }],
+      ['border-top-style', {
+        'values': [border_style, 'inherit'],
+        'initial': null,
+      }],
+      ['border-right-style', {
+        'values': [border_style, 'inherit'],
+        'initial': null,
+      }],
+      ['border-bottom-style', {
+        'values': [border_style, 'inherit'],
+        'initial': null,
+      }],
+      ['border-left-style', {
+        'values': [border_style, 'inherit'],
+        'initial': null,
+      }],
+      ['border-top-width', {
+        'values': ['thin', 'medium', 'thick', length, 'inherit'],
+        'initial': 'medium',
+      }],
+      ['border-right-width', {
+        'values': ['thin', 'medium', 'thick', length, 'inherit'],
+        'initial': 'medium',
+      }],
+      ['border-bottom-width', {
+        'values': ['thin', 'medium', 'thick', length, 'inherit'],
+        'initial': 'medium',
+      }],
+      ['border-left-width', {
+        'values': ['thin', 'medium', 'thick', length, 'inherit'],
+        'initial': 'medium',
+      }],
+      //border-width
+      //border
+      ['bottom', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['caption-side', {
+        'values': ['top', 'bottom', 'inherit'],
+        'initial': 'top',
+      }],
+      ['clear', {
+        'values': ['none', 'left', 'right', 'both', 'inherit'],
+        'initial': 'none',
+      }],
+      ['clip', {
+        'values': [shape, 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['color', {
+        'values': [color, 'inherit'],
+        'initial': 'black', //FIXME depends on user agent
+      }],
+      ['content', {
+        'values': ['normal', 'none', string, uri, counter, attr, 'inherit'], //FIXME
+        'initial': 'normal',
+      }],
+      //counter-increment
+      //counter-reset
+      ['cursor', {
+        'values': [ 'auto', 'crosshair', 'default', 'pointer', 'move', 'e-resize', 'ne-resize',
+                    'nw-resize', 'n-resize', 'se-resize', 'sw-resize', 's-resize', 'w-resize',
+                    'text', 'wait', 'help', 'progress', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['direction', {
+        'values': ['ltr', 'rtl', 'inherit'],
+        'initial': 'ltr',
+      }],
+      ['display', {
+        'values': ['inline', 'block', 'list-item', 'inline-block', 'table', 'inline-table',
+                   'table-row-group', 'table-header-group', 'table-footer-group', 'table-row',
+                   'table-column-group', 'table-column', 'table-cell', 'table-caption', 'none',
+                   'inherit'],
+        'initial': 'inline',
+      }],
+      ['empty-cells', {
+         'values': ['show', 'hide', 'inherit'],
+         'initial': 'show',
+      }],
+      ['float', {
+        'values': ['left', 'right', 'none', 'inherit'],
+        'initial': 'none',
+        'property': 'cssFloat',
+      }],
+      ['font-family', {
+        'values': [family_name, generic_family, 'inherit'],
+        'initial': 'sans-serif', //FIXME depends on user agent
+      }],
+      ['font-size', {
+        'values': [absolute_size, relative_size, length, percentage, 'inherit'],
+        'initial': 'medium',
+      }],
+      ['font-style', {
+        'values': ['normal', 'italic', 'oblique', 'inherit'],
+        'initial': 'normal',
+      }],
+      ['font-variant', {
+        'values': ['normal', 'small-caps', 'inherit'],
+        'initial': 'normal',
+      }],
+      ['font-weight', {
+        'values': ['normal', 'bold', 'bolder', 'lighter', 100, 200, 300, 400, 500, 600,
+                   700, 800, 900, 'inherit'],
+        'initial': 'normal',
+      }],
+      //font
+      ['height', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['left', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['letter-spacing', {
+        'values': ['normal', length, 'inherit'],
+        'initial': 'normal',
+      }],
+      ['line-height', {
+        'values': ['normal', positive_number, length, percentage, 'inherit'],
+        'initial': 'normal',
+      }],
+      ['list-style-image', {
+        'values': [uri, 'none', 'inherit'],
+        'initial': 'none',
+      }],
+      ['list-style-position', {
+        'values': ['inside', 'outside', 'inherit'],
+        'initial': 'outside',
+      }],
+      ['list-style-type', {
+        'values': ['disc', 'circle', 'square', 'disclosure-open', 'disclosure-closed',
+                   'decimal', 'decimal-leading-zero', 'lower-roman',
+                   'upper-roman', 'lower-greek', 'lower-latin', 'upper-latin', 'armenian', 'georgian',
+                   'lower-alpha', 'upper-alpha', 'none', 'inherit'],
+        'initial': 'disc',
+      }],
+      //list-style
+      ['margin-right', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 0,
+      }],
+      ['margin-left', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 0,
+      }],
+      ['margin-top', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 0,
+      }],
+      ['margin-bottom', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 0,
+      }],
+      //margin
+      ['max-height', {
+        'values': [length, percentage, 'none', 'inherit'],
+        'initial': 'none',
+      }],
+      ['max-width', {
+        'values': [length, percentage, 'none', 'inherit'],
+        'initial': 'none',
+      }],
+      ['min-height', {
+        'values': [length, percentage, 'inherit'],
+        'initial': 0,
+      }],
+      ['min-width', {
+        'values': [length, percentage, 'inherit'],
+        'initial': 0,
+      }],
+      ['orphans', {
+        'values': [nonzero_positive_integer, 'inherit'],
+        'initial': 2,
+      }],
+      ['outline-color', {
+        'values': [color, 'invert', 'inherit'],
+        'initial': 'invert',
+      }],
+      ['outline-style', {
+        'values': [border_style_without_hidden, 'inherit'],
+        'initial': 'none',
+      }],
+      ['outline-width', {
+        'values': ['thin', 'medium', 'thick', length, 'inherit'],
+        'initial': 'medium',
+      }],
+      //outline
+      ['overflow', {
+        'values': ['visible', 'hidden', 'scroll', 'auto', 'inherit'],
+        'initial': 'visible',
+      }],
+      ['padding-top', {
+        'values': [length, percentage, 'inherit'],
+        'initial': 0,
+      }],
+      ['padding-right', {
+        'values': [length, percentage, 'inherit'],
+        'initial': 0,
+      }],
+      ['padding-bottom', {
+        'values': [length, percentage, 'inherit'],
+        'initial': 0,
+      }],
+      ['padding-left', {
+        'values': [length, percentage, 'inherit'],
+        'initial': 0,
+      }],
+      //padding
+      ['page-break-after', {
+        'values': ['auto', 'always', 'avoid', 'left', 'right', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['page-break-before', {
+        'values': ['auto', 'always', 'avoid', 'left', 'right', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['page-break-inside', {
+        'values': ['avoid', 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['position', {
+        'values': ['static', 'relative', 'absolute', 'fixed', 'inherit'],
+        'initial': 'static',
+      }],
+      //FIXME quotes
+      ['right', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['table-layout', {
+        'values': ['auto', 'fixed', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['text-align', {
+        'values': ['left', 'right', 'center', 'justify', 'inherit'],
+        'initial': null,
+      }],
+      ['text-decoration', {
+        'values': ['none', 'underline', 'overline', 'line-through', 'blink', 'inherit'],
+        'initial': 'none',
+      }],
+      ['text-indent', {
+        'values': [length, percentage, 'inherit'],
+        'initial': 0,
+      }],
+      ['text-transform', {
+        'values': ['capitalize', 'uppercase', 'lowercase', 'none', 'inherit'],
+        'initial': 'none',
+      }],
+      ['top', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['unicode-bidi', {
+        'values': ['normal', 'embed', 'bidi-override', 'inherit'],
+        'initial': 'normal',
+      }],
+      ['vertical-align', {
+        'values': ['baseline', 'sub', 'super', 'top', 'text-top', 'middle', 'bottom', 'text-bottom',
+                   percentage, length, 'inherit'],
+        'initial': 'baseline',
+      }],
+      ['visibility', {
+        'values': ['visible', 'hidden', 'collapse', 'inherit'],
+        'initial': 'visible',
+      }],
+      ['white-space', {
+        'values': ['normal', 'pre', 'nowrap', 'pre-wrap', 'pre-line', 'inherit'],
+        'initial': 'normal',
+      }],
+      ['widows', {
+        'values': [nonzero_positive_integer, 'inherit'],
+        'initial': 2,
+      }],
+      ['width', {
+        'values': [length, percentage, 'auto', 'inherit'],
+        'initial': 'auto',
+      }],
+      ['word-spacing', {
+        'values': ['normal', length, 'inherit'],
+        'initial': 'normal',
+      }],
+      ['z-index', {
+        'values': ['auto', integer, 'inherit'],
+        'initial': 'auto',
+      }],
+    ]
+
+    for (var index = 0; index < properties.length; index++) {
+      test_property(properties[index]);
+    }
+  </script>
+</body>
diff --git a/testing/web-platform/tests/css/cssom/serialize-variable-reference.html b/testing/web-platform/tests/css/cssom/serialize-variable-reference.html
new file mode 100644
index 0000000000..1d3cbad9e6
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/serialize-variable-reference.html
@@ -0,0 +1,36 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSSOM - Serialization with variable preserves original serialization.</title>
+<link rel="help" href="https://drafts.csswg.org/css-variables/#serializing-custom-props">
+<link rel="help" href="https://drafts.csswg.org/css-variables/#variables-in-shorthands">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="longhand-whitespace" style="font-size: var(--a);"></div>
+<div id="shorthand-whitespace" style="font: var(--a);"></div>
+<div id="longhand" style="font-size:var(--a);"></div>
+<div id="shorthand" style="font:var(--a);"></div>
+<script>
+    test(function() {
+        var elem = document.getElementById('longhand-whitespace');
+
+        assert_equals(elem.style.cssText, 'font-size: var(--a);');
+    }, 'Longhand with variable preserves original serialization: with whitespace')
+
+    test(function() {
+        var elem = document.getElementById('shorthand-whitespace');
+
+        assert_equals(elem.style.cssText, 'font: var(--a);');
+    }, 'Shorthand with variable preserves original serialization: with whitespace')
+
+    test(function() {
+        var elem = document.getElementById('longhand');
+
+        assert_equals(elem.style.cssText, 'font-size: var(--a);');
+    }, 'Longhand with variable preserves original serialization but trims whitespace: without whitespace')
+
+    test(function() {
+        var elem = document.getElementById('shorthand');
+
+        assert_equals(elem.style.cssText, 'font: var(--a);');
+    }, 'Shorthand with variable preserves original serialization but trims whitespace: without whitespace')
+</script>
diff --git a/testing/web-platform/tests/css/cssom/setproperty-null-undefined.html b/testing/web-platform/tests/css/cssom/setproperty-null-undefined.html
new file mode 100644
index 0000000000..3de142c0b4
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/setproperty-null-undefined.html
@@ -0,0 +1,47 @@
+<!doctype html>
+<html>
+<head>
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty"/>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script>
+var style = document.body.style;
+
+test(function() {
+    style.color = 'white';
+
+    assert_equals(style.color, 'white');
+    style.setProperty('color', undefined);
+    assert_equals(style.color, 'white');
+}, "Verify that setting a CSS property to undefined has no effect.");
+
+test(function() {
+    style.color = 'white';
+
+    assert_equals(style.color, 'white');
+    assert_equals(style.getPropertyPriority('color'), '');
+    style.setProperty('color', 'red', undefined);
+    assert_equals(style.color, 'red');
+    assert_equals(style.getPropertyPriority('color'), '');
+}, "Verify that setting a CSS property priority to undefined is accepted.");
+
+test(function() {
+    style.color = 'white';
+
+    assert_equals(style.color, 'white');
+    style.setProperty('color', null);
+    assert_equals(style.color, '');
+}, "Verify that setting a CSS property to null is treated like empty string.");
+
+test(function() {
+    style.color = 'white';
+
+    assert_equals(style.color, 'white');
+    style.setProperty('color', 'red', null);
+    assert_equals(style.color, 'red');
+}, "Verify that setting a CSS property priority to null is treated like empty string.");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/shorthand-serialization.html b/testing/web-platform/tests/css/cssom/shorthand-serialization.html
new file mode 100644
index 0000000000..97e11da8b8
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/shorthand-serialization.html
@@ -0,0 +1,86 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>Shorthand serialization should be done correctly.</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-declaration-block">
+    <link rel="help" href="https://drafts.csswg.org/css-variables/#variables-in-shorthands">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+    <div id="foo1" style="background: red;">foo</div>
+    <div id="foo2" style="background-color: blue; background: red !important; background-color: green;">foo</div>
+    <div id="foo3" style="background-color: blue; background: red; background-color: green !important;">foo</div>
+
+    <div id="foo4" style="margin-right: 10px; margin-left: 10px; margin-top: 10px; margin-bottom: 10px;">foo</div>
+    <div id="foo5" style="margin-right: 10px; margin-left: 10px; margin-top: 10px; margin-bottom: 10px!important;">foo</div>
+    <div id="foo6" style="margin-right: 10px !important; margin-left: 10px !important; margin-top: 10px !important; margin-bottom: 10px!important;">foo</div>
+
+    <div id="foo7" style="background:var(--a);">foo</div>
+    <div id="test"></div>
+
+    <script>
+        test(function() {
+            var elem1 = document.getElementById('foo1');
+            var elem2 = document.getElementById('foo2');
+            var elem3 = document.getElementById('foo3');
+
+            assert_false(elem1.style.cssText.endsWith('!important;'));
+            assert_true(elem2.style.cssText.endsWith('!important;'));
+            assert_false(elem3.style.background.endsWith('!important'));
+
+        }, "Shorthand serialization with shorthand and longhands mixed.");
+
+        test(function() {
+            var elem4 = document.getElementById('foo4');
+            var elem5 = document.getElementById('foo5');
+            var elem6 = document.getElementById('foo6');
+
+            assert_equals(elem4.style.cssText, 'margin: 10px;');
+            assert_equals(elem4.style.margin, '10px');
+            assert_equals(elem5.style.cssText, 'margin-right: 10px; margin-left: 10px; margin-top: 10px; margin-bottom: 10px !important;');
+            assert_equals(elem5.style.margin, '');
+            assert_equals(elem6.style.cssText, 'margin: 10px !important;');
+            assert_equals(elem6.style.margin, '10px');
+        }, "Shorthand serialization with just longhands.");
+
+        test(function() {
+          var elem7 = document.getElementById('foo7');
+
+          assert_equals(elem7.style.background, 'var(--a)');
+          assert_equals(elem7.style.backgroundPosition, '');
+        }, "Shorthand serialization with variable and variable from other shorthand.");
+
+        test(function() {
+            var testElem = document.getElementById("test");
+            testElem.style.margin = "20px 20px 20px 20px";
+            assert_equals(testElem.style.margin, "20px");
+            assert_equals(testElem.style.cssText, "margin: 20px;")
+        }, "Shorthand serialization after setting");
+
+        test(function() {
+            const testElem = document.getElementById("test");
+            testElem.style.cssText = "margin: initial;";
+            assert_equals(testElem.style.margin, "initial");
+            assert_equals(testElem.style.cssText, "margin: initial;");
+        }, "Shorthand serialization with 'initial' value.");
+
+        test(function() {
+            const testElem = document.getElementById("test");
+            testElem.style.setProperty("margin-top", "initial", "important");
+            assert_equals(testElem.style.margin, "");
+        }, "Shorthand serialization with 'initial' value, one longhand with important flag.");
+
+        test(function() {
+            const testElem = document.getElementById("test");
+            testElem.style.cssText = "";
+            testElem.style.setProperty("margin-top", "initial");
+            testElem.style.setProperty("margin-right", "initial");
+            testElem.style.setProperty("margin-bottom", "initial");
+            testElem.style.setProperty("margin-left", "initial", "important");
+            assert_equals(testElem.style.margin, "");
+        }, "Shorthand serialization with 'initial' value, longhands set individually, one with important flag.");
+    </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/shorthand-values.html b/testing/web-platform/tests/css/cssom/shorthand-values.html
new file mode 100644
index 0000000000..b64f7e9a12
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/shorthand-values.html
@@ -0,0 +1,51 @@
+<!doctype html>
+<head>
+  <title>CSS OM: CSS Values</title>
+  <link rel="author" title="Divya Manian" href="mailto:manian@adobe.com">
+  <link rel="help" href="https://drafts.csswg.org/cssom/#serialize-a-css-declaration-block">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="Testing Serialization of Shorthand Values">
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+    <div id="test"></div>
+    <script>
+      function test_shorthand_serialization(value, expected) {
+        test(function() {
+          const div = document.getElementById("test");
+          div.style.cssText = value;
+          assert_equals(div.style.cssText, expected);
+        }, "The serialization of " + value + " should be canonical.");
+      }
+
+      var tests = {
+        // specified -> expected
+        'border: 1px; border-top: 1px;': 'border: 1px;',
+        'border: 1px solid red;': 'border: 1px solid red;',
+        'border: 1px red;': 'border: 1px red;',
+        'border: red;': 'border: red;',
+        'border-top: 1px; border-right: 1px; border-bottom: 1px; border-left: 1px; border-image: none;': 'border: 1px;',
+        'border-top: 1px; border-right: 1px; border-bottom: 1px; border-left: 1px;': 'border-width: 1px; border-style: none; border-color: currentcolor;',
+        'border-top: 1px; border-right: 2px; border-bottom: 3px; border-left: 4px;': 'border-width: 1px 2px 3px 4px; border-style: none; border-color: currentcolor;',
+        'border: 1px; border-top: 2px;': 'border-width: 2px 1px 1px; border-style: none; border-color: currentcolor; border-image: none;',
+        'border: 1px; border-top: 1px !important;': 'border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-right-style: none; border-bottom-style: none; border-left-style: none; border-right-color: currentcolor; border-bottom-color: currentcolor; border-left-color: currentcolor; border-image: none; border-top-width: 1px !important; border-top-style: none !important; border-top-color: currentcolor !important;',
+        'border: 1px; border-top-color: red;': 'border-width: 1px; border-style: none; border-color: red currentcolor currentcolor; border-image: none;',
+        'border: solid; border-style: dotted': 'border: dotted;',
+        'border-width: 1px;': 'border-width: 1px;',
+        'overflow-x: scroll; overflow-y: hidden;': 'overflow: scroll hidden;',
+        'overflow-x: scroll; overflow-y: scroll;': 'overflow: scroll;',
+        'outline-width: 2px; outline-style: dotted; outline-color: blue;': 'outline: blue dotted 2px;',
+        'margin-top: 1px; margin-right: 2px; margin-bottom: 3px; margin-left: 4px;': 'margin: 1px 2px 3px 4px;',
+        'list-style-type: circle; list-style-position: inside; list-style-image: none;': 'list-style: inside circle;',
+        'list-style-type: lower-alpha;': 'list-style-type: lower-alpha;',
+        'font-family: sans-serif; line-height: 2em; font-size: 3em; font-style: italic; font-weight: bold;': 'font-family: sans-serif; line-height: 2em; font-size: 3em; font-style: italic; font-weight: bold;',
+        'padding-top: 1px; padding-right: 2px; padding-bottom: 3px; padding-left: 4px;': 'padding: 1px 2px 3px 4px;'
+      }
+
+      for (let test in tests) {
+        test_shorthand_serialization(test, tests[test]);
+      }
+    </script>
+ </body>
+ </html>
diff --git a/testing/web-platform/tests/css/cssom/style-attr-update-across-documents.html b/testing/web-platform/tests/css/cssom/style-attr-update-across-documents.html
new file mode 100644
index 0000000000..2b8f443520
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/style-attr-update-across-documents.html
@@ -0,0 +1,48 @@
+<!DOCTYPE html>
+<title>CSSStyleDeclaration setter works when a node changes document</title>
+<link rel="author" title="Raphael Kubo da Costa" href="raphael.kubo.da.costa@intel.com">
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-camel-cased-attribute">
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-webkit-cased-attribute">
+<link rel="help" href="https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-dashed-attribute">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="original-div" style="background-color:green; height: 100px; width: 100px"></div>
+<script>
+  // Create and return a 100x100 red <div>.
+  function createDiv(containerDocument) {
+    const div = containerDocument.createElement("div");
+    div.style.backgroundColor = "red";
+    div.style.height = "100px";
+    div.style.width = "100px";
+    containerDocument.body.appendChild(div);
+    return div;
+  }
+
+  async_test(t => {
+    const iframeElement = document.createElement("iframe");
+    iframeElement.addEventListener("load", t.step_func_done(() => {
+      const originalDiv = document.getElementById("original-div");
+      const newDiv = createDiv(iframeElement.contentDocument);
+
+      assert_not_equals(newDiv.ownerDocument, document,
+                        "The new <div> belongs to the iframe, not the main document");
+
+      document.body.insertBefore(newDiv, originalDiv);
+      assert_equals(newDiv.ownerDocument, document,
+                    "The new <div> now belongs to the main document");
+
+      newDiv.style.backgroundColor = "blue";
+      assert_equals(window.getComputedStyle(newDiv).getPropertyValue("background-color"),
+                    "rgb(0, 0, 255)",
+                    "The new <div>'s background-color is blue");
+
+      document.body.removeChild(iframeElement);
+
+      newDiv.style.backgroundColor = "green";
+      assert_equals(window.getComputedStyle(newDiv).getPropertyValue("background-color"),
+                    "rgb(0, 128, 0)",
+                    "The new <div>'s background-color is green");
+    }));
+    document.body.appendChild(iframeElement);
+  }, "Changing the style of a node that switched documents works");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/style-sheet-interfaces-001.html b/testing/web-platform/tests/css/cssom/style-sheet-interfaces-001.html
new file mode 100644
index 0000000000..07c4fbb41a
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/style-sheet-interfaces-001.html
@@ -0,0 +1,140 @@
+<!DOCTYPE html>
+<html>
+ <head>
+  <title>CSS Test: CSSOM StyleSheet Initial Values</title>
+  <link rel="author" title="Bear Travis" href="mailto:betravis@adobe.com">
+  <link rel="help" href="https://www.w3.org/TR/cssom-1/#css-style-sheets">
+  <link rel="help" href="https://www.w3.org/TR/cssom-1/#the-cssimportrule-interface">
+  <link rel="help" href="https://www.w3.org/TR/cssom-1/#the-linkstyle-interface">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="StyleSheet and CSSStyleSheet objects have the properties specified in their interfaces">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+  <style id="styleElement" type="text/css" media="all" title="internal style sheet" disabled="disabled">
+    @import url('support/a-green.css');
+    * { margin: 0; padding: 0; }
+  </style>
+  <link id="linkElement" rel="stylesheet" href="support/b-green.css">
+ </head>
+ <body>
+  <noscript>Test not run - javascript required.</noscript>
+  <div id="log"></div>
+  <script type="text/javascript">
+    var styleElement = document.getElementById("styleElement");
+    var linkElement = document.getElementById("linkElement");
+
+    var styleSheet;
+    var linkSheet;
+
+    // styleElement.sheet exists and is a CSSStyleSheet
+    // linkElement.sheet exists and is a CSSStyleSheet
+    test(function() {
+        assert_idl_attribute(styleElement, "sheet");
+        assert_readonly(styleElement, "sheet");
+        styleSheet = styleElement.sheet;
+        assert_true(styleSheet instanceof CSSStyleSheet);
+        assert_idl_attribute(linkElement, "sheet");
+        linkSheet = linkElement.sheet;
+        assert_true(linkSheet instanceof CSSStyleSheet);
+    }, "sheet_property");
+
+    // The `disabled` IDL attribute setter has no effect before the underlying
+    // CSSStyleSheet is created.
+    test(function () {
+      const style = document.createElement("style");
+      style.disabled = true;
+      assert_false(style.disabled);
+      document.head.append(style);
+      const sheet1 = style.sheet;
+      assert_false(style.disabled);
+      assert_false(sheet1.disabled);
+
+      // The `disabled` attribute can be set after the sheet exists.
+      style.disabled = true;
+      assert_true(style.disabled);
+      assert_true(sheet1.disabled);
+
+      // Reset `disabled`.
+      style.disabled = false;
+      assert_false(style.disabled);
+      assert_false(style.disabled);
+
+      // Setting `disabled = true` on the CSSStyleSheet also carries over to the
+      // HTMLStyleElement.
+      sheet1.disabled = true;
+      assert_true(style.disabled);
+      assert_true(sheet1.disabled);
+    }, "disabled attribute getter/setter");
+
+    // The sheet property on LinkStyle should always return the current associated style sheet.
+    test(function () {
+      var style = document.createElement("style");
+      document.querySelector("head").appendChild(style);
+      var sheet1 = style.sheet;
+      assert_equals(sheet1.cssRules.length, 0);
+      style.appendChild(document.createTextNode("a { color: green; }"));
+      assert_equals(style.sheet.cssRules.length, 1);
+    }, "sheet_property_updates");
+
+    // ownerRule, cssRules, insertRule and deleteRule properties exist on CSSStyleSheet
+    // ownerRule, cssRules are read only
+    test(function() {
+        assert_idl_attribute(styleSheet, "ownerRule");
+        assert_idl_attribute(styleSheet, "cssRules");
+        assert_inherits(styleSheet, "insertRule");
+        assert_inherits(styleSheet, "deleteRule");
+
+        assert_readonly(styleSheet, "ownerRule");
+        assert_readonly(styleSheet, "cssRules");
+    }, "CSSStyleSheet_properties");
+
+    var importSheet;
+    // CSSStyleSheet initial property values are correct
+    test(function() {
+        assert_equals(styleSheet.ownerRule, null);
+        assert_true(styleSheet.cssRules.length > 0);
+        assert_true(styleSheet.cssRules.item(0) instanceof CSSImportRule);
+        importSheet = styleSheet.cssRules.item(0).styleSheet;
+    }, "CSSStyleSheet_property_values");
+
+    // type, disabled, ownerNode, parentStyleSheet, href, title, and media properties exist on StyleSheet
+    // type, ownerNode, parentStyleSheet, href, and title properties are read only
+    test(function() {
+        assert_idl_attribute(styleSheet, "type");
+        assert_idl_attribute(styleSheet, "disabled");
+        assert_idl_attribute(styleSheet, "ownerNode");
+        assert_idl_attribute(styleSheet, "parentStyleSheet");
+        assert_idl_attribute(styleSheet, "href");
+        assert_idl_attribute(styleSheet, "title");
+        assert_idl_attribute(styleSheet, "media");
+
+        assert_readonly(styleSheet, "type");
+        assert_readonly(styleSheet, "ownerNode");
+        assert_readonly(styleSheet, "parentStyleSheet");
+        assert_readonly(styleSheet, "href");
+        assert_readonly(styleSheet, "title");
+    }, "StyleSheet_properties");
+
+    // StyleSheet initial property values are correct
+    test(function() {
+        assert_equals(styleSheet.type, "text/css");
+        assert_equals(styleSheet.disabled, false);
+
+        assert_equals(styleSheet.ownerNode, styleElement);
+        assert_equals(linkSheet.ownerNode, linkElement);
+        assert_equals(importSheet.ownerNode, null);
+
+        assert_equals(styleSheet.href, null);
+        assert_regexp_match(linkSheet.href, /support\/b-green.css$/);
+        assert_regexp_match(importSheet.href, /support\/a-green.css$/);
+
+        assert_equals(styleSheet.parentStyleSheet, null);
+        assert_equals(linkSheet.parentStyleSheet, null);
+        assert_equals(importSheet.parentStyleSheet, styleSheet);
+
+        assert_equals(styleSheet.title, "internal style sheet");
+        assert_equals(styleSheet.media.item(0), "all");
+    }, "StyleSheet_property_values");
+  </script>
+ </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/style-sheet-interfaces-002.html b/testing/web-platform/tests/css/cssom/style-sheet-interfaces-002.html
new file mode 100644
index 0000000000..e86a9a16e3
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/style-sheet-interfaces-002.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html>
+ <head>
+  <title>CSS Test: CSSOM StyleSheet Modify Rule List</title>
+  <link rel="author" title="Bear Travis" href="mailto:betravis@adobe.com">
+  <link rel="reviewer" title="Ms2ger" href="mailto:ms2ger@gmail.com"> <!-- 2012-06-17 -->
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssstylesheet-interface">
+  <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-cssrule-interface">
+  <meta name="flags" content="dom">
+  <meta name="assert" content="StyleSheet and CSSStyleSheet objects have the properties specified in their interfaces">
+  <script src="/resources/testharness.js" type="text/javascript"></script>
+  <script src="/resources/testharnessreport.js" type="text/javascript"></script>
+  <style id="styleElement" type="text/css" media="all" title="internal style sheet" disabled="disabled">
+    * { margin: 0; padding: 0; }
+  </style>
+ </head>
+ <body>
+  <noscript>Test not run - javascript required.</noscript>
+  <div id="log"></div>
+  <script type="text/javascript">
+  var sheet = document.getElementById("styleElement").sheet;
+  // Initial rule list is of size 1
+  // Can add a rule at first index
+  test(function() {
+    assert_equals(sheet.cssRules.length, 1);
+    sheet.insertRule("p { color: green; }", 0);
+    assert_equals(sheet.cssRules.length, 2);
+    assert_equals(sheet.cssRules.item(0).cssText, "p { color: green; }");
+  }, "add_rule");
+
+  // Can delete rules until rule list is empty
+  test(function() {
+    sheet.deleteRule(0);
+    assert_equals(sheet.cssRules.length, 1);
+    sheet.deleteRule(0);
+    assert_equals(sheet.cssRules.length, 0);
+  }, "delete_rule");
+  </script>
+ </body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/stylesheet-replacedata-dynamic-ref.html b/testing/web-platform/tests/css/cssom/stylesheet-replacedata-dynamic-ref.html
new file mode 100644
index 0000000000..bc9cadebf1
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/stylesheet-replacedata-dynamic-ref.html
@@ -0,0 +1,7 @@
+<!doctype html>
+<title>(Ref #1) CSS Test Reference</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@mozilla.com">
+<link rel="author" title="Cheng You Bai" href="mailto:cyb.ai.815@gmail.com">
+<link rel="help" href="https://dom.spec.whatwg.org/#dom-characterdata-replacedata">
+<style>.pass { color: green }</style>
+<div class="pass">Should be green</div>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/stylesheet-replacedata-dynamic.html b/testing/web-platform/tests/css/cssom/stylesheet-replacedata-dynamic.html
new file mode 100644
index 0000000000..3ee5937c95
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/stylesheet-replacedata-dynamic.html
@@ -0,0 +1,12 @@
+<!doctype html>
+<title>(Test #1) CSS Test: Dynamic changes to the stylesheet contents using replaceData are reflected</title>
+<link rel="match" href="stylesheet-replacedata-dynamic-ref.html">
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@mozilla.com">
+<link rel="author" title="Cheng You Bai" href="mailto:cyb.ai.815@gmail.com">
+<link rel="help" href="https://dom.spec.whatwg.org/#dom-characterdata-replacedata">
+<style>.fail { color: green }</style>
+<div class="pass">Should be green</div>
+<script>
+  document.body.offsetTop;
+  document.querySelector('style').firstChild.replaceData(1, 4, "pass");
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/stylesheet-same-origin.css b/testing/web-platform/tests/css/cssom/stylesheet-same-origin.css
new file mode 100644
index 0000000000..d61a8dd67c
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/stylesheet-same-origin.css
@@ -0,0 +1,3 @@
+body {
+    padding: 10px;
+}
diff --git a/testing/web-platform/tests/css/cssom/stylesheet-same-origin.sub.html b/testing/web-platform/tests/css/cssom/stylesheet-same-origin.sub.html
new file mode 100644
index 0000000000..6ad55190da
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/stylesheet-same-origin.sub.html
@@ -0,0 +1,83 @@
+<!doctype html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>CSSOM - CSSStylesheet should support origins</title>
+    <link rel="help" href="https://drafts.csswg.org/cssom/#the-cssstylesheet-interface">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+
+    <link id="crossorigin" href="http://www1.{{host}}:{{ports[http][1]}}/css/cssom/stylesheet-same-origin.css" rel="stylesheet">
+    <link id="sameorigin" href="stylesheet-same-origin.css" rel="stylesheet">
+    <link id="sameorigindata" href="data:text/css,.green-text{color:rgb(0, 255, 0)}" rel="stylesheet">
+    <link id="redirect-sameorigin-to-crossorigin"
+          href="/common/redirect.py?location=http://www1.{{host}}:{{ports[http][1]}}/css/cssom/stylesheet-same-origin.css"
+          rel="stylesheet">
+    <link id="redirect-crossorigin-to-sameorigin"
+          href="http://www1.{{host}}:{{ports[http][1]}}/common/redirect.py?location=http://{{host}}:{{ports[http][0]}}/css/cssom/stylesheet-same-origin.css"
+          rel="stylesheet">
+    <link id="loaderror" href="support/malformed-http-response.asis" rel="stylesheet">
+
+    <script>
+        var crossorigin = document.getElementById("crossorigin").sheet;
+        var redirectSameOriginToCrossOrigin = document.getElementById("redirect-sameorigin-to-crossorigin").sheet;
+        var redirectCrossOriginToSameOrigin = document.getElementById("redirect-crossorigin-to-sameorigin").sheet;
+        var loadError = document.getElementById("loaderror").sheet;
+        var sameorigin = document.getElementById("sameorigin").sheet;
+        var sameorigindata = document.getElementById("sameorigindata").sheet;
+
+        function doOriginCleanCheck(sheet, name) {
+            assert_equals(sheet.cssRules.length, 1, name + " stylesheet.cssRules should be accessible.");
+            sheet.insertRule("#test { margin: 10px; }", 1);
+            assert_equals(sheet.cssRules.length, 2, name + " stylesheet.insertRule should be accessible.");
+            sheet.deleteRule(0);
+            assert_equals(sheet.cssRules.length, 1, name + " stylesheet.deleteRule should be accessible.");
+        }
+
+        function doOriginDirtyCheck(sheet) {
+            assert_throws_dom("SecurityError",
+                function () {
+                    sheet.cssRules;
+                },
+                'stylesheet.cssRules should throw SecurityError.');
+            assert_throws_dom("SecurityError",
+                function () {
+                    sheet.insertRule("#test { margin: 10px; }", 1);
+                },
+                'stylesheet.insertRule should throw SecurityError.');
+
+            assert_throws_dom("SecurityError",
+                function () {
+                    sheet.deleteRule(0);
+                },
+                'stylesheet.deleteRule should throw SecurityError.');
+        }
+
+        test(function() {
+            doOriginDirtyCheck(crossorigin);
+        }, "Origin-clean check in cross-origin CSSOM Stylesheets");
+
+        test(function() {
+            doOriginDirtyCheck(redirectSameOriginToCrossOrigin);
+        }, "Origin-clean check in cross-origin CSSOM Stylesheets (redirect from same-origin to cross-origin)");
+
+        test(function() {
+            doOriginDirtyCheck(redirectCrossOriginToSameOrigin);
+        }, "Origin-clean check in cross-origin CSSOM Stylesheets (redirect from cross-origin to same-origin)");
+
+       test(function() {
+            doOriginDirtyCheck(loadError);
+        }, "Origin-clean check in loading error CSSOM Stylesheets");
+
+        test(function() {
+            doOriginCleanCheck(sameorigin, "Same-origin");
+        }, "Origin-clean check in same-origin CSSOM Stylesheets");
+
+        test(function() {
+            doOriginCleanCheck(sameorigindata, "data:css");
+        }, "Origin-clean check in data:css CSSOM Stylesheets");
+
+    </script>
+</head>
+<body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/stylesheet-title.html b/testing/web-platform/tests/css/cssom/stylesheet-title.html
new file mode 100644
index 0000000000..77b1df9677
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/stylesheet-title.html
@@ -0,0 +1,39 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: StyleSheet's title attribute</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://drafts.csswg.org/cssom/#preferred-css-style-sheet-set-name">
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#attr-style-title">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style></style>
+<style title=""></style>
+<style title="Preferred">
+  p { color: green; }
+</style>
+<style title="Not preferred">
+  p { color: red; }
+</style>
+<p id="test-element">Should be green</p>
+<script>
+test(function() {
+  assert_equals(
+    getComputedStyle(document.getElementById("test-element")).color,
+    "rgb(0, 128, 0)",
+    "Preferred style should apply"
+  );
+}, "Preferred style sheet name");
+
+test(function() {
+  let sheets = document.styleSheets;
+  let styleElements = Array.from(document.querySelectorAll("style"));
+  assert_equals(sheets.length, styleElements.length);
+  for (let i = 0; i < sheets.length; ++i) {
+    let titleAttr = styleElements[i].getAttribute("title");
+    if (titleAttr === null || titleAttr === "")
+      assert_equals(sheets[i].title, null, "Empty title returns null");
+    else
+      assert_equals(sheets[i].title, titleAttr, "Selected title is properly reflected");
+  }
+}, "StyleSheet.title");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/support/1x1-green.png b/testing/web-platform/tests/css/cssom/support/1x1-green.png
new file mode 100644
index 0000000000..b98ca0ba0a
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/1x1-green.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/1x1-lime.png b/testing/web-platform/tests/css/cssom/support/1x1-lime.png
new file mode 100644
index 0000000000..cb397fb090
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/1x1-lime.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/1x1-maroon.png b/testing/web-platform/tests/css/cssom/support/1x1-maroon.png
new file mode 100644
index 0000000000..3f86b07219
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/1x1-maroon.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/1x1-navy.png b/testing/web-platform/tests/css/cssom/support/1x1-navy.png
new file mode 100644
index 0000000000..9b9a03955b
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/1x1-navy.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/1x1-red.png b/testing/web-platform/tests/css/cssom/support/1x1-red.png
new file mode 100644
index 0000000000..6bd73ac101
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/1x1-red.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/1x1-white.png b/testing/web-platform/tests/css/cssom/support/1x1-white.png
new file mode 100644
index 0000000000..dd43faec54
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/1x1-white.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/60x60-gg-rr.png b/testing/web-platform/tests/css/cssom/support/60x60-gg-rr.png
new file mode 100644
index 0000000000..84f5b2a4f1
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/60x60-gg-rr.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/60x60-green.png b/testing/web-platform/tests/css/cssom/support/60x60-green.png
new file mode 100644
index 0000000000..b3c8cf3eb4
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/60x60-green.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/README b/testing/web-platform/tests/css/cssom/support/README
new file mode 100644
index 0000000000..2e5f2ad073
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/README
@@ -0,0 +1,28 @@
+CSS Global Support Directory
+============================
+
+This directory contains common support files (such as images and external
+style sheets). These are sync'ed into the support directories of all our
+test suites. If you have test-suite-specific support files, please add
+them to the appropriate test-suite-specific support/ directory.
+
+If you add to a support/ directory, please run the tools/supportprop.py
+script from the top of the repository to cascade support files into the
+lower-level support directories.
+
+Description of the Common Support File Collection
+-------------------------------------------------
+
+The 1x1-* images are all exactly one pixel.
+
+The swatch-* images all use 15x15 cells.
+
+The square-* images all use 15x15 cells with one pixel borders.
+
+The pattern-* images use cells of various sizes:
+
+   pattern-grg-rgr-grg.png   20x20
+   pattern-rgr-grg-rgr.png   20x20
+   pattern-tr.png            15x15
+   pattern-grg-rrg-rgg.png   15x15
+
diff --git a/testing/web-platform/tests/css/cssom/support/a-green.css b/testing/web-platform/tests/css/cssom/support/a-green.css
new file mode 100644
index 0000000000..b0dbb071d5
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/a-green.css
@@ -0,0 +1 @@
+.a { color: green; }
diff --git a/testing/web-platform/tests/css/cssom/support/b-green.css b/testing/web-platform/tests/css/cssom/support/b-green.css
new file mode 100644
index 0000000000..a0473f5ca2
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/b-green.css
@@ -0,0 +1 @@
+.b { color: green; }
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/support/black.css b/testing/web-platform/tests/css/cssom/support/black.css
new file mode 100644
index 0000000000..833a0e0092
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/black.css
@@ -0,0 +1,4 @@
+html {
+background-color: black;
+color: white;
+}
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/support/c-red.css b/testing/web-platform/tests/css/cssom/support/c-red.css
new file mode 100644
index 0000000000..d4ba5c64e9
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/c-red.css
@@ -0,0 +1 @@
+.c { color: red; }
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/support/cat.png b/testing/web-platform/tests/css/cssom/support/cat.png
new file mode 100644
index 0000000000..85dd732481
Binary files /dev/null and b/testing/web-platform/tests/css/cssom/support/cat.png differ
diff --git a/testing/web-platform/tests/css/cssom/support/constructable-import.css b/testing/web-platform/tests/css/cssom/support/constructable-import.css
new file mode 100644
index 0000000000..d8ccc9d491
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/constructable-import.css
@@ -0,0 +1,3 @@
+* {
+  color: red;
+}
diff --git a/testing/web-platform/tests/css/cssom/support/getComputedStyle-insets.js b/testing/web-platform/tests/css/cssom/support/getComputedStyle-insets.js
new file mode 100644
index 0000000000..723990cafb
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/getComputedStyle-insets.js
@@ -0,0 +1,375 @@
+export const testEl = document.createElement("div");
+export const containerForInflow = document.createElement("div");
+export const containerForAbspos = document.createElement("div");
+export const containerForFixed = document.createElement("div");
+
+testEl.id = "test";
+containerForInflow.id = "container-for-inflow";
+containerForAbspos.id = "container-for-abspos";
+containerForFixed.id = "container-for-fixed";
+
+containerForInflow.appendChild(testEl);
+containerForAbspos.appendChild(containerForInflow);
+containerForFixed.appendChild(containerForAbspos);
+document.body.appendChild(containerForFixed);
+
+const stylesheet = document.createElement("style");
+stylesheet.textContent = `
+  #container-for-inflow {
+    /* Content area: 100px tall, 200px wide */
+    height: 100px;
+    width: 200px;
+    padding: 1px 2px;
+    border-width: 2px 4px;
+    margin: 4px 8px;
+  }
+  #container-for-abspos {
+    /* Padding area: 200px tall, 400px wide */
+    height: 184px;
+    width: 368px;
+    padding: 8px 16px;
+    border-width: 16px 32px;
+    margin: 32px 64px;
+    position: relative;
+  }
+  #container-for-fixed {
+    /* Padding area: 300px tall, 600px wide */
+    height: 172px;
+    width: 344px;
+    padding: 64px 128px;
+    border-width: 128px 256px;
+    margin: 256px 512px;
+    position: absolute;
+    transform: scale(1);
+    visibility: hidden;
+  }
+  [id ^= container] {
+    border-style: solid;
+  }
+`;
+document.head.prepend(stylesheet);
+
+function runTestsWithWM(data, testWM, cbWM) {
+  const {
+    style,
+    containingBlockElement,
+    containingBlockArea,
+    preservesPercentages,
+    preservesAuto,
+    canStretchAutoSize,
+    staticPositionX,
+    staticPositionY,
+  } = data;
+
+  let cbHeight = containingBlockElement ? containingBlockElement.clientHeight : NaN;
+  let cbWidth = containingBlockElement ? containingBlockElement.clientWidth : NaN;
+  if (containingBlockElement && containingBlockArea == "content") {
+    const cs = getComputedStyle(containingBlockElement);
+    cbHeight -= parseFloat(cs.paddingTop) + parseFloat(cs.paddingBottom);
+    cbWidth -= parseFloat(cs.paddingLeft) + parseFloat(cs.paddingRight);
+  }
+
+  const staticPositionTop = cbWM.blockStart == "top" || cbWM.inlineStart == "top"
+    ? staticPositionY : cbHeight - staticPositionY;
+  const staticPositionLeft = cbWM.blockStart == "left" || cbWM.inlineStart == "left"
+    ? staticPositionX : cbWidth - staticPositionX;
+  const staticPositionBottom = cbWM.blockStart == "bottom" || cbWM.inlineStart == "bottom"
+    ? staticPositionY : cbHeight - staticPositionY;
+  const staticPositionRight = cbWM.blockStart == "right" || cbWM.inlineStart == "right"
+    ? staticPositionX : cbWidth - staticPositionX;
+
+  function serialize(declarations) {
+    return Object.entries(declarations).map(([p, v]) => `${p}: ${v}; `).join("");
+  }
+
+  function wmName(wm) {
+    return Object.values(wm.style).join(" ");
+  }
+
+  function checkStyle(declarations, expected, msg) {
+    test(function() {
+      testEl.style.cssText = style + "; " + serialize(Object.assign({}, declarations, testWM.style));
+      if (containingBlockElement) {
+        containingBlockElement.style.cssText = serialize(Object.assign({}, cbWM.style));
+      }
+      const cs = getComputedStyle(testEl);
+      for (let [prop, value] of Object.entries(expected)) {
+        assert_equals(cs[prop], value, `'${prop}'`);
+      }
+    }, `${wmName(testWM)} inside ${wmName(cbWM)} - ${msg}`);
+
+    testEl.style.cssText = "";
+    if (containingBlockElement) {
+      containingBlockElement.style.cssText = "";
+    }
+  }
+
+  checkStyle({
+    top: "1px",
+    left: "2px",
+    bottom: "3px",
+    right: "4px",
+  }, {
+    top: "1px",
+    left: "2px",
+    bottom: "3px",
+    right: "4px",
+  }, "Pixels resolve as-is");
+
+  checkStyle({
+    top: "1em",
+    left: "2em",
+    bottom: "3em",
+    right: "4em",
+    "font-size": "10px",
+  }, {
+    top: "10px",
+    left: "20px",
+    bottom: "30px",
+    right: "40px",
+  }, "Relative lengths are absolutized into pixels");
+
+  if (preservesPercentages) {
+    checkStyle({
+      top: "10%",
+      left: "25%",
+      bottom: "50%",
+      right: "75%",
+    }, {
+      top: "10%",
+      left: "25%",
+      bottom: "50%",
+      right: "75%",
+    }, "Percentages resolve as-is");
+  } else {
+    checkStyle({
+      top: "10%",
+      left: "25%",
+      bottom: "50%",
+      right: "75%",
+    }, {
+      top: cbHeight * 10 / 100 + "px",
+      left: cbWidth * 25 / 100 + "px",
+      bottom: cbHeight * 50 / 100 + "px",
+      right: cbWidth * 75 / 100 + "px",
+    }, "Percentages are absolutized into pixels");
+
+    checkStyle({
+      top: "calc(10% - 1px)",
+      left: "calc(25% - 2px)",
+      bottom: "calc(50% - 3px)",
+      right: "calc(75% - 4px)",
+    }, {
+      top: cbHeight * 10 / 100 - 1 + "px",
+      left: cbWidth * 25 / 100 - 2 + "px",
+      bottom: cbHeight * 50 / 100 - 3 + "px",
+      right: cbWidth * 75 / 100 - 4 + "px",
+    }, "calc() is absolutized into pixels");
+  }
+
+  if (canStretchAutoSize) {
+    // Force overconstraintment by setting size or with insets that would result in
+    // negative size. Then the resolved value should be the computed one according to
+    // https://drafts.csswg.org/cssom/#resolved-value-special-case-property-like-top
+
+    checkStyle({
+      top: "1px",
+      left: "2px",
+      bottom: "3px",
+      right: "4px",
+      height: "0px",
+      width: "0px",
+    }, {
+      top: "1px",
+      left: "2px",
+      bottom: "3px",
+      right: "4px",
+    }, "Pixels resolve as-is when overconstrained");
+
+    checkStyle({
+      top: "100%",
+      left: "100%",
+      bottom: "100%",
+      right: "100%",
+    }, {
+      top: cbHeight + "px",
+      left: cbWidth + "px",
+      bottom: cbHeight + "px",
+      right: cbWidth + "px",
+    }, "Percentages absolutize the computed value when overconstrained");
+  }
+
+  if (preservesAuto) {
+    checkStyle({
+      top: "auto",
+      left: "auto",
+      bottom: "3px",
+      right: "4px",
+    }, {
+      top: "auto",
+      left: "auto",
+      bottom: "3px",
+      right: "4px",
+    }, "If start side is 'auto' and end side is not, 'auto' resolves as-is");
+
+    checkStyle({
+      top: "1px",
+      left: "2px",
+      bottom: "auto",
+      right: "auto",
+    }, {
+      top: "1px",
+      left: "2px",
+      bottom: "auto",
+      right: "auto",
+    }, "If end side is 'auto' and start side is not, 'auto' resolves as-is");
+
+    checkStyle({
+      top: "auto",
+      left: "auto",
+      bottom: "auto",
+      right: "auto",
+    }, {
+      top: "auto",
+      left: "auto",
+      bottom: "auto",
+      right: "auto",
+    }, "If opposite sides are 'auto', they resolve as-is");
+  } else if (canStretchAutoSize) {
+    checkStyle({
+      top: "auto",
+      left: "auto",
+      bottom: "3px",
+      right: "4px",
+    }, {
+      top: cbHeight - 3 + "px",
+      left: cbWidth - 4 + "px",
+      bottom: "3px",
+      right: "4px",
+    }, "If start side is 'auto' and end side is not, 'auto' resolves to used value");
+
+    checkStyle({
+      top: "1px",
+      left: "2px",
+      bottom: "auto",
+      right: "auto",
+    }, {
+      top: "1px",
+      left: "2px",
+      bottom: cbHeight - 1 + "px",
+      right: cbWidth - 2 + "px",
+    }, "If end side is 'auto' and start side is not, 'auto' resolves to used value");
+
+    checkStyle({
+      top: "auto",
+      left: "auto",
+      bottom: "auto",
+      right: "auto",
+    }, {
+      top: staticPositionTop + "px",
+      left: staticPositionLeft + "px",
+      bottom: staticPositionBottom + "px",
+      right: staticPositionRight + "px",
+    }, "If opposite sides are 'auto', they resolve to used value");
+  } else {
+    checkStyle({
+      top: "auto",
+      left: "auto",
+      bottom: "3px",
+      right: "4px",
+    }, {
+      top: "-3px",
+      left: "-4px",
+      bottom: "3px",
+      right: "4px",
+    }, "If start side is 'auto' and end side is not, 'auto' resolves to used value");
+
+    checkStyle({
+      top: "1px",
+      left: "2px",
+      bottom: "auto",
+      right: "auto",
+    }, {
+      top: "1px",
+      left: "2px",
+      bottom: "-1px",
+      right: "-2px",
+    }, "If end side is 'auto' and start side is not, 'auto' resolves to used value");
+
+    checkStyle({
+      top: "auto",
+      left: "auto",
+      bottom: "auto",
+      right: "auto",
+    }, {
+      top: "0px",
+      left: "0px",
+      bottom: "0px",
+      right: "0px",
+    }, "If opposite sides are 'auto', they resolve to used value");
+  }
+}
+
+const writingModes = [{
+  style: {
+    "writing-mode": "horizontal-tb",
+    "direction": "ltr",
+  },
+  blockStart: "top",
+  blockEnd: "bottom",
+  inlineStart: "left",
+  inlineEnd: "right",
+}, {
+  style: {
+    "writing-mode": "horizontal-tb",
+    "direction": "rtl",
+  },
+  blockStart: "top",
+  blockEnd: "bottom",
+  inlineStart: "right",
+  inlineEnd: "left",
+}, {
+  style: {
+    "writing-mode": "vertical-lr",
+    "direction": "ltr",
+  },
+  blockStart: "left",
+  blockEnd: "right",
+  inlineStart: "top",
+  inlineEnd: "bottom",
+}, {
+  style: {
+    "writing-mode": "vertical-lr",
+    "direction": "rtl",
+  },
+  blockStart: "left",
+  blockEnd: "right",
+  inlineStart: "bottom",
+  inlineEnd: "top",
+}, {
+  style: {
+    "writing-mode": "vertical-rl",
+    "direction": "ltr",
+  },
+  blockStart: "right",
+  blockEnd: "left",
+  inlineStart: "top",
+  inlineEnd: "bottom",
+}, {
+  style: {
+    "writing-mode": "vertical-rl",
+    "direction": "rtl",
+  },
+  blockStart: "right",
+  blockEnd: "left",
+  inlineStart: "bottom",
+  inlineEnd: "top",
+}];
+
+export function runTests(data) {
+  for (let testWM of writingModes) {
+    for (let cbWM of writingModes) {
+      runTestsWithWM(data, testWM, cbWM);
+    }
+  }
+}
diff --git a/testing/web-platform/tests/css/cssom/support/import-charset.css b/testing/web-platform/tests/css/cssom/support/import-charset.css
new file mode 100644
index 0000000000..9f44090c94
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/import-charset.css
@@ -0,0 +1 @@
+@charset "UTF-8";
diff --git a/testing/web-platform/tests/css/cssom/support/import-green.css b/testing/web-platform/tests/css/cssom/support/import-green.css
new file mode 100644
index 0000000000..537104e663
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/import-green.css
@@ -0,0 +1 @@
+.import { color: green; }
diff --git a/testing/web-platform/tests/css/cssom/support/import-red.css b/testing/web-platform/tests/css/cssom/support/import-red.css
new file mode 100644
index 0000000000..9945ef4711
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/import-red.css
@@ -0,0 +1 @@
+.import { color: red; }
diff --git a/testing/web-platform/tests/css/cssom/support/import-rule.css b/testing/web-platform/tests/css/cssom/support/import-rule.css
new file mode 100644
index 0000000000..5edcef7e69
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/import-rule.css
@@ -0,0 +1 @@
+@import "import-red.css";
diff --git a/testing/web-platform/tests/css/cssom/support/malformed-http-response.asis b/testing/web-platform/tests/css/cssom/support/malformed-http-response.asis
new file mode 100644
index 0000000000..bc3c68d46d
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/malformed-http-response.asis
@@ -0,0 +1 @@
+HAHAHA THIS IS NOT HTTP AND THE BROWSER SHOULD CONSIDER IT A NETWORK ERROR
diff --git a/testing/web-platform/tests/css/cssom/support/pattern-grg-rgr-grg.png b/testing/web-platform/tests/css/cssom/support/pattern-grg-rgr-grg.png
new file mode 100644
index 0000000000..9b88fbd811
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diff --git a/testing/web-platform/tests/css/cssom/support/pattern-grg-rrg-rgg.png b/testing/web-platform/tests/css/cssom/support/pattern-grg-rrg-rgg.png
new file mode 100644
index 0000000000..fcf4f3fd7d
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diff --git a/testing/web-platform/tests/css/cssom/support/pattern-rgr-grg-rgr.png b/testing/web-platform/tests/css/cssom/support/pattern-rgr-grg-rgr.png
new file mode 100644
index 0000000000..d454e3a630
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diff --git a/testing/web-platform/tests/css/cssom/support/pattern-tr.png b/testing/web-platform/tests/css/cssom/support/pattern-tr.png
new file mode 100644
index 0000000000..8b4b25364e
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diff --git a/testing/web-platform/tests/css/cssom/support/ruler-h-50%.png b/testing/web-platform/tests/css/cssom/support/ruler-h-50%.png
new file mode 100644
index 0000000000..cf2eea6b43
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diff --git a/testing/web-platform/tests/css/cssom/support/ruler-h-50px.png b/testing/web-platform/tests/css/cssom/support/ruler-h-50px.png
new file mode 100644
index 0000000000..9f46583665
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diff --git a/testing/web-platform/tests/css/cssom/support/ruler-v-100px.png b/testing/web-platform/tests/css/cssom/support/ruler-v-100px.png
new file mode 100644
index 0000000000..a837eca222
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diff --git a/testing/web-platform/tests/css/cssom/support/ruler-v-50px.png b/testing/web-platform/tests/css/cssom/support/ruler-v-50px.png
new file mode 100644
index 0000000000..8414102802
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diff --git a/testing/web-platform/tests/css/cssom/support/square-purple.png b/testing/web-platform/tests/css/cssom/support/square-purple.png
new file mode 100644
index 0000000000..0f522d7872
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diff --git a/testing/web-platform/tests/css/cssom/support/square-teal.png b/testing/web-platform/tests/css/cssom/support/square-teal.png
new file mode 100644
index 0000000000..e567f51b91
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diff --git a/testing/web-platform/tests/css/cssom/support/square-white.png b/testing/web-platform/tests/css/cssom/support/square-white.png
new file mode 100644
index 0000000000..5853cbb238
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diff --git a/testing/web-platform/tests/css/cssom/support/support/README b/testing/web-platform/tests/css/cssom/support/support/README
new file mode 100644
index 0000000000..ea8cb9ef35
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/support/README
@@ -0,0 +1,4 @@
+The swatch-green.png file in this directory is really a RED swatch,
+and the swatch-red.png file is really a green swatch.
+
+This directory is used to test relative URIs.
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/cssom/support/support/swatch-green.png b/testing/web-platform/tests/css/cssom/support/support/swatch-green.png
new file mode 100644
index 0000000000..1caf25c992
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diff --git a/testing/web-platform/tests/css/cssom/support/support/swatch-red.png b/testing/web-platform/tests/css/cssom/support/support/swatch-red.png
new file mode 100644
index 0000000000..0aa79b0c86
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-blue.png b/testing/web-platform/tests/css/cssom/support/swatch-blue.png
new file mode 100644
index 0000000000..bf2759634d
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-green.png b/testing/web-platform/tests/css/cssom/support/swatch-green.png
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-lime.png b/testing/web-platform/tests/css/cssom/support/swatch-lime.png
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index 0000000000..55fd7fdaed
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-orange.png b/testing/web-platform/tests/css/cssom/support/swatch-orange.png
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index 0000000000..d3cd498b52
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-red.png b/testing/web-platform/tests/css/cssom/support/swatch-red.png
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index 0000000000..1caf25c992
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-teal.png b/testing/web-platform/tests/css/cssom/support/swatch-teal.png
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index 0000000000..0293ce89de
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-white.png b/testing/web-platform/tests/css/cssom/support/swatch-white.png
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index 0000000000..1a7d4323d7
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diff --git a/testing/web-platform/tests/css/cssom/support/swatch-yellow.png b/testing/web-platform/tests/css/cssom/support/swatch-yellow.png
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diff --git a/testing/web-platform/tests/css/cssom/support/test-bl.png b/testing/web-platform/tests/css/cssom/support/test-bl.png
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index 0000000000..904e24e996
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diff --git a/testing/web-platform/tests/css/cssom/support/test-br.png b/testing/web-platform/tests/css/cssom/support/test-br.png
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index 0000000000..f413ff5c1a
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diff --git a/testing/web-platform/tests/css/cssom/support/test-inner-half-size.png b/testing/web-platform/tests/css/cssom/support/test-inner-half-size.png
new file mode 100644
index 0000000000..e473bf80ef
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diff --git a/testing/web-platform/tests/css/cssom/support/test-outer.png b/testing/web-platform/tests/css/cssom/support/test-outer.png
new file mode 100644
index 0000000000..82eeace7fc
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diff --git a/testing/web-platform/tests/css/cssom/support/test-tl.png b/testing/web-platform/tests/css/cssom/support/test-tl.png
new file mode 100644
index 0000000000..f6ac0ef7e8
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diff --git a/testing/web-platform/tests/css/cssom/support/test-tr.png b/testing/web-platform/tests/css/cssom/support/test-tr.png
new file mode 100644
index 0000000000..59843ae54b
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diff --git a/testing/web-platform/tests/css/cssom/support/xmlss-pi.xhtml b/testing/web-platform/tests/css/cssom/support/xmlss-pi.xhtml
new file mode 100644
index 0000000000..8e22b50b25
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/support/xmlss-pi.xhtml
@@ -0,0 +1 @@
+<?xml-stylesheet href='data:text/css,'?><html xmlns='http://www.w3.org/1999/xhtml'/>
diff --git a/testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-count.html b/testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-count.html
new file mode 100644
index 0000000000..5c2b9e6f46
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-count.html
@@ -0,0 +1,61 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSSOM - Extensions to the Document Interface: StyleSheetList length reflects dynamically loaded and unloaded sheets</title>
+    <link rel="author" title="Jesse Bounds" href="mailto:jesse@codeforamerica.org">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#extensions-to-the-document-interface">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-stylesheetlist-interface">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#css-style-sheet-collections">
+    <link rel="stylesheet" href="stylesheet.css" type="text/css">
+    <meta name="flags" content="dom">
+    <meta name="assert" content="The styleSheets length attribute must reflect the number of sheets at page load and after dynamically">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+    <div id="log"></div>
+    <script>
+
+      // Get the Document's styleSheets attribute
+      var styleSheets = document.styleSheets;
+
+      // Verify that the styleSheets list length is 1 due to "stylesheet.css" loaded in head section
+      test(function() {
+        assert_equals(styleSheets.length, 1, "styleSheets.length is incorrect:");
+      }, "stylesheet.css should be loaded and styleSheets.length === 1");
+
+      // Verify that the styleSheets list length is 0 after removing the loaded sheet from the DOM
+      test(function() {
+
+        // get the one and only sheet loaded
+        var sheet = styleSheets.item(0);
+
+        // remove the sheet from the DOM
+        sheet.ownerNode.parentNode.removeChild(sheet.ownerNode)
+
+        // assert that there are 0 styleSheets in the styleSheets property
+        assert_equals(styleSheets.length, 0, "styleSheets.length is incorrect:");
+
+      }, "stylesheet.css should be unloaded and styleSheets.length === 0");
+
+      // Verify that the styleSheets list length is back to 1 after loading a new sheet
+      async_test(function(t) {
+
+        // create a css file reference
+        var fileReference = document.createElement("link");
+        fileReference.setAttribute("rel", "stylesheet");
+        fileReference.setAttribute("type", "text/css");
+        fileReference.setAttribute("href", "stylesheet-1.css");
+        fileReference.onerror = t.step_func_done(function() {
+          // assert that there is 1 styleSheet in the styleSheets property
+          assert_equals(styleSheets.length, 1, "styleSheets.length is incorrect:");
+        });
+
+        // load the css file reference into the head section
+        var head = document.getElementsByTagName("HEAD")[0];
+        head.appendChild(fileReference);
+      }, "stylesheet-1.css should be loaded and styleSheets.length === 1");
+
+    </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-tree-order.html b/testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-tree-order.html
new file mode 100644
index 0000000000..2824401cb2
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/ttwf-cssom-doc-ext-load-tree-order.html
@@ -0,0 +1,56 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSSOM -  Extensions to the Document Interface: Stylesheet header load order</title>
+    <link rel="author" title="Jesse Bounds" href="mailto:jesse@codeforamerica.org">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#extensions-to-the-document-interface">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-stylesheetlist-interface">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#css-style-sheet-collections">
+    <style title="aaa" type="text/css">
+      H1 {border-width: 1; border: solid; text-align: center}
+    </style>
+    <link title="zebra" rel="stylesheet" href="zebra.css" type="text/css">
+    <link title="kilo" rel="stylesheet" href="kilo.css"  type="text/css">
+    <link title="alpha" rel="stylesheet" href="alpha.css" type="text/css">
+    <link title="zebra" rel="stylesheet" href="/directory01/zebra.css" type="text/css">
+    <meta name="flags" content="dom">
+    <meta name="assert" content="Document's style sheets created from HTTP Link headers are first in list and loaded in header order">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+    <div id="log"></div>
+    <script>
+    onload = function() {
+      // Get the Document's styleSheets attribute
+      var styleSheets = document.styleSheets;
+
+      // Verify that the styleSheets list length is 5
+      test(function() {
+        assert_equals(styleSheets.length, 5, "styleSheets.length is incorrect:");
+      }, "styleSheets.length must be 5");
+
+      // Verify that titles of loaded sheets are as expected (in the correct order)
+      test(function() {
+        assert_equals(styleSheets.item(0).title, "aaa", "title for item 1 is incorrect:");
+      }, "styleSheets item 0 title must be aaa");
+      // Verify that titles of loaded sheets are as expected (in the correct order)
+      test(function() {
+        assert_equals(styleSheets.item(1).title, "zebra", "title for item 1 is incorrect:");
+      }, "styleSheets item 1 title must be zebra");
+      // Verify that titles of loaded sheets are as expected (in the correct order)
+      test(function() {
+        assert_equals(styleSheets.item(2).title, "kilo", "title for item 1 is incorrect:");
+      }, "styleSheets item 0 title must be kilo");
+      // Verify that titles of loaded sheets are as expected (in the correct order)
+      test(function() {
+        assert_equals(styleSheets.item(3).title, "alpha", "title for item 1 is incorrect:");
+      }, "styleSheets item 0 title must be alpha");
+      // Verify that titles of loaded sheets are as expected (in the correct order)
+      test(function() {
+        assert_equals(styleSheets.item(4).title, "zebra", "title for item 1 is incorrect:");
+      }, "styleSheets item 0 title must be zebra");
+    };
+    </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/ttwf-cssom-document-extension.html b/testing/web-platform/tests/css/cssom/ttwf-cssom-document-extension.html
new file mode 100644
index 0000000000..0c9116dc4b
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/ttwf-cssom-document-extension.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>CSSOM - Extensions to the Document Interface: StyleSheetList length is 0 when no sheets loaded</title>
+    <link rel="author" title="Jesse Bounds" href="mailto:jesse@codeforamerica.org">
+    <link rel="reviewer" title="Ms2ger" href="mailto:ms2ger@gmail.com"> <!-- 2012-06-17 -->
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#extensions-to-the-document-interface">
+    <link rel="help" href="http://www.w3.org/TR/cssom-1/#the-stylesheetlist-interface">
+    <meta name="flags" content="dom">
+    <meta name="assert" content="The styleSheets attribute must return a StyleSheetList sequence representing the document style sheets.">
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+    <div id="log"></div>
+    <script>
+        // Get the Document's styleSheets attribute
+        var styleSheets = document.styleSheets;
+        // Verify that the styleSheets list length is 0 because no stylesheets have been loaded
+        test(function() {
+          assert_equals(styleSheets.length, 0, "styleSheets.length is incorrect:");
+        })
+    </script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/css/cssom/variable-names.html b/testing/web-platform/tests/css/cssom/variable-names.html
new file mode 100644
index 0000000000..5591411e6e
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/variable-names.html
@@ -0,0 +1,48 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Tests for handling of CSS Custom Property names</title>
+<link rel="help" href="https://drafts.csswg.org/css-variables/#serializing-custom-props">
+<meta name="author" title="Cameron McCormack" href="mailto:cam@mcc.id.au">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+
+// Valid custom property names, before and after CSS escaping.
+var valid_names = [
+  ["--a", "--a"],
+  ["--a;b", "--a\\;b"],
+  ["---", "---"],
+  ["--\\", "--\\\\"],
+  ["--ab", "--\\61 b"],
+  ["--0", "--\\30 "],
+];
+
+valid_names.forEach(function(t) {
+  var name = t[0];
+  var escaped_name = t[1];
+
+  test(function() {
+    var e = document.createElement("span");
+    e.style = escaped_name + ":value";
+
+    for (var after_refeeding = 0; after_refeeding <= 1; ++after_refeeding) {
+      var desc_suffix = (after_refeeding ? " (after " : " (before ") +
+                        "serialization/re-parsing)";
+
+      assert_equals(e.style.length, 1,
+                    "appears on specified style" + desc_suffix);
+
+      assert_equals(e.style[0], name,
+                    "name returned correctly from specified " +
+                    "style indexed getter" + desc_suffix);
+
+      assert_equals(e.style.getPropertyValue(name), "value",
+                    "property value returned correctly from " +
+                    "specified style getPropertyValue" + desc_suffix);
+
+      e.style = e.style.cssText;
+    }
+  }, "custom property '" + name + "'");
+});
+</script>
diff --git a/testing/web-platform/tests/css/cssom/xml-stylesheet-pi-in-doctype.xhtml b/testing/web-platform/tests/css/cssom/xml-stylesheet-pi-in-doctype.xhtml
new file mode 100644
index 0000000000..d25ca17656
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/xml-stylesheet-pi-in-doctype.xhtml
@@ -0,0 +1,17 @@
+<!DOCTYPE html [<?xml-stylesheet href="data:text/css,html{z-index: 1}"?>]>
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <title>xml-stylesheet processing instruction in doctype internal subset</title>
+    <link rel="help" href="https://w3c.github.io/csswg-drafts/cssom/#prolog"/>
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+  </head>
+  <body>
+    <script>
+      promise_test(async () => {
+        await new Promise(resolve => window.onload = resolve);
+        assert_equals(getComputedStyle(document.documentElement).zIndex, "auto");
+      });
+    </script>
+  </body>
+</html>
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