From 0ebf5bdf043a27fd3dfb7f92e0cb63d88954c44d Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Fri, 19 Apr 2024 03:47:29 +0200
Subject: Adding upstream version 115.8.0esr.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../web-platform/tests/css/css-layout-api/META.yml |   4 +
 .../css/css-layout-api/at-supports-rule.https.html |  23 +++
 .../auto-block-size/absolute.https.html            |  73 ++++++++
 .../auto-block-size-absolute-ref.html              |  27 +++
 .../auto-block-size-floats-ref.html                |  26 +++
 .../auto-block-size-negative-ref.html              |  11 ++
 .../css-layout-api/auto-block-size/flex-ref.html   |  21 +++
 .../css-layout-api/auto-block-size/flex.https.html |  61 +++++++
 .../auto-block-size/floats.https.html              |  57 ++++++
 .../css-layout-api/auto-block-size/inflow-ref.html |  13 ++
 .../auto-block-size/inflow.https.html              |  63 +++++++
 .../auto-block-size/negative.https.html            |  50 ++++++
 .../baseline/child-baseline.https.html             |  58 ++++++
 .../baseline/flex-baseline.https.html              |  51 ++++++
 .../css-layout-api/baseline/no-baseline.https.html |  42 +++++
 .../baseline/orthogonal-baseline.https.html        |  42 +++++
 .../css-layout-api/box-tree-registered-ref.html    |  20 +++
 .../css-layout-api/box-tree-registered.https.html  |  66 +++++++
 .../css-layout-api/box-tree-unregistered-ref.html  |  34 ++++
 .../box-tree-unregistered.https.html               |  62 +++++++
 .../available-block-size-htb-vrl.https.html        |  65 +++++++
 .../available-block-size-invalid.https.html        |  49 ++++++
 .../available-block-size-vrl-htb.https.html        |  65 +++++++
 .../available-inline-size-htb-htb.https.html       |  65 +++++++
 .../available-inline-size-invalid.https.html       |  49 ++++++
 .../available-inline-size-vrl-vrl.https.html       |  65 +++++++
 ...ailable-size-for-percentages-htb-htb.https.html |  59 +++++++
 ...ailable-size-for-percentages-htb-vrl.https.html |  59 +++++++
 ...ailable-size-for-percentages-invalid.https.html |  49 ++++++
 ...ailable-size-for-percentages-vrl-htb.https.html |  59 +++++++
 ...ailable-size-for-percentages-vrl-vrl.https.html |  59 +++++++
 .../fixed-block-size-vrl.https.html                |  60 +++++++
 .../child-constraints/fixed-block-size.https.html  |  59 +++++++
 .../fixed-inline-size-vrl.https.html               |  60 +++++++
 .../child-constraints/fixed-inline-size.https.html |  59 +++++++
 .../percentage-size-htb-htb.https.html             |  58 ++++++
 .../percentage-size-htb-vrl.https.html             |  58 ++++++
 .../percentage-size-invalid.https.html             |  47 +++++
 .../percentage-size-quirks-mode.https.html         |  55 ++++++
 .../percentage-size-vrl-htb.https.html             |  58 ++++++
 .../percentage-size-vrl-vrl.https.html             |  58 ++++++
 .../support/layout-child-sizes-worklet.js          |  67 +++++++
 .../chrome-bug-1287843-000-crash.https.html        |  12 ++
 .../chrome-bug-1287843-001-crash.https.html        |  18 ++
 .../chrome-bug-1291449-crash.https.html            |  11 ++
 .../chrome-bug-1296664-crash.https.html            |  18 ++
 .../computed-style-layout-function.https.html      |  50 ++++++
 .../constraints-data-function-failure.https.html   |  61 +++++++
 .../constraints-data-sab-failure.https.html        |  62 +++++++
 .../css/css-layout-api/constraints-data.https.html |  69 ++++++++
 ...d-block-size-absolute-left-right-vrl.https.html |  40 +++++
 .../fixed-block-size-absolute-none.https.html      |  36 ++++
 ...fixed-block-size-absolute-top-bottom.https.html |  39 +++++
 .../fixed-block-size-block-none-vrl.https.html     |  32 ++++
 .../fixed-block-size-block-none.https.html         |  32 ++++
 .../fixed-block-size-fixed-max.https.html          |  34 ++++
 .../fixed-block-size-fixed-min.https.html          |  34 ++++
 .../fixed-block-size-fixed-vrl.https.html          |  33 ++++
 .../constraints/fixed-block-size-fixed.https.html  |  33 ++++
 .../fixed-block-size-flex-basis-vrl.https.html     |  37 ++++
 .../fixed-block-size-flex-column-basis.https.html  |  38 ++++
 .../fixed-block-size-flex-column-grow.https.html   |  40 +++++
 .../fixed-block-size-flex-column-none.https.html   |  37 ++++
 ...d-block-size-flex-column-stretch-vrl.https.html |  39 +++++
 .../fixed-block-size-flex-grow-vrl.https.html      |  38 ++++
 .../fixed-block-size-flex-none.https.html          |  36 ++++
 ...lock-size-flex-percentage-indefinite.https.html |  37 ++++
 .../fixed-block-size-flex-stretch-max.https.html   |  39 +++++
 .../fixed-block-size-flex-stretch.https.html       |  38 ++++
 .../fixed-block-size-grid-none.https.html          |  36 ++++
 .../fixed-block-size-grid-stretch-max.https.html   |  39 +++++
 .../fixed-block-size-grid-stretch.https.html       |  38 ++++
 .../fixed-block-size-grid-vrl.https.html           |  38 ++++
 ...xed-block-size-percentage-indefinite.https.html |  33 ++++
 ...ed-block-size-percentage-quirks-mode.https.html |  38 ++++
 .../fixed-block-size-percentage-vrl.https.html     |  37 ++++
 .../fixed-block-size-percentage.https.html         |  37 ++++
 .../fixed-block-size-quirky-body.https.html        |  22 +++
 ...ixed-inline-size-absolute-left-right.https.html |  33 ++++
 ...-inline-size-absolute-top-bottom-vrl.https.html |  35 ++++
 ...ine-size-block-auto-avoid-floats-vlr.https.html |  37 ++++
 ...-inline-size-block-auto-avoid-floats.https.html |  36 ++++
 .../fixed-inline-size-block-auto-vlr.https.html    |  31 ++++
 .../fixed-inline-size-block-auto.https.html        |  30 ++++
 .../fixed-inline-size-fixed-vrl.https.html         |  27 +++
 .../constraints/fixed-inline-size-fixed.https.html |  26 +++
 ...xed-inline-size-flex-grow-column-vrl.https.html |  33 ++++
 .../fixed-inline-size-flex-grow.https.html         |  31 ++++
 .../constraints/fixed-inline-size-grid.https.html  |  30 ++++
 .../fixed-inline-size-percentage-vlr.https.html    |  31 ++++
 .../fixed-inline-size-percentage.https.html        |  30 ++++
 .../constraints/fixed-inline-size-ref.html         |  10 ++
 ...raints-fixed-block-size-quirky-body-iframe.html |  21 +++
 .../support/constraints-fixed-block-size.js        |  22 +++
 .../support/constraints-fixed-inline-size.js       |   9 +
 .../css/css-layout-api/crash-multicol.https.html   |  28 +++
 .../tests/css/css-layout-api/edges/all-ref.html    | 161 +++++++++++++++++
 .../tests/css/css-layout-api/edges/all.https.html  | 194 +++++++++++++++++++++
 .../css-layout-api/edges/border-htb-rtl.https.html |  39 +++++
 .../css/css-layout-api/edges/border-htb.https.html |  38 ++++
 .../css-layout-api/edges/border-vlr-rtl.https.html |  39 +++++
 .../css/css-layout-api/edges/border-vlr.https.html |  38 ++++
 .../css-layout-api/edges/border-vrl-rtl.https.html |  39 +++++
 .../css/css-layout-api/edges/border-vrl.https.html |  38 ++++
 .../edges/padding-htb-rtl.https.html               |  37 ++++
 .../css-layout-api/edges/padding-htb.https.html    |  36 ++++
 .../edges/padding-vlr-rtl.https.html               |  37 ++++
 .../css-layout-api/edges/padding-vlr.https.html    |  36 ++++
 .../edges/padding-vrl-rtl.https.html               |  37 ++++
 .../css-layout-api/edges/padding-vrl.https.html    |  36 ++++
 .../css/css-layout-api/edges/scrollbar-ref.html    | 159 +++++++++++++++++
 .../css/css-layout-api/edges/scrollbar.https.html  | 192 ++++++++++++++++++++
 .../css/css-layout-api/edges/support/edges.js      |  38 ++++
 .../fallback-intrinsic-sizes/bad-return.https.html |  49 ++++++
 .../constructor-error.https.html                   |  50 ++++++
 .../fallback-intrinsic-sizes/error.https.html      |  49 ++++++
 .../fallback-intrinsic-sizes/fallback-ref.html     |  11 ++
 .../invalid-child.https.html                       |  84 +++++++++
 .../fallback-intrinsic-sizes/no-promise.https.html |  41 +++++
 .../unresolved-promise.https.html                  |  41 +++++
 .../fallback-layout/bad-return.https.html          |  50 ++++++
 .../fallback-layout/constructor-error.https.html   |  51 ++++++
 .../fallback-layout/error.https.html               |  50 ++++++
 .../fallback-layout-fallback-ref.html              |  11 ++
 .../fallback-layout/invalid-child.https.html       |  84 +++++++++
 .../fallback-layout/invalid-fragment.https.html    |  82 +++++++++
 .../fallback-layout/no-promise.https.html          |  41 +++++
 .../fallback-layout/unresolved-promise.https.html  |  41 +++++
 .../fragment-data-function-failure.https.html      |  43 +++++
 .../fragment-data-immutable.https.html             |  69 ++++++++
 .../fragment-data-sab-failure.https.html           |  45 +++++
 .../css/css-layout-api/fragment-data.https.html    |  64 +++++++
 .../tests/css/css-layout-api/green-square-ref.html |   9 +
 .../inline-style-layout-function.https.html        |  36 ++++
 .../css/css-layout-api/input-text-crash.https.html |   4 +
 .../inside-multicol-crash.https.html               |   8 +
 .../intrinsic-sizes/absolute-ref.html              |  43 +++++
 .../intrinsic-sizes/absolute.https.html            |  73 ++++++++
 .../child-replaced-percentage-01.https.html        |  47 +++++
 .../child-replaced-percentage-02.https.html        |  49 ++++++
 .../child-replaced-percentage-ref.html             |   9 +
 .../intrinsic-sizes/child-size-01-ref.html         |   9 +
 .../intrinsic-sizes/child-size-01.https.html       |  61 +++++++
 .../intrinsic-sizes/child-size-02-ref.html         |   9 +
 .../intrinsic-sizes/child-size-02.https.html       |  67 +++++++
 .../intrinsic-sizes/child-size-03.https.html       |  67 +++++++
 .../child-size-contribution.https.html             |  49 ++++++
 .../intrinsic-sizes/content-size-ref.html          |  11 ++
 .../intrinsic-sizes/content-size.https.html        |  48 +++++
 .../css-layout-api/intrinsic-sizes/floats-ref.html |  47 +++++
 .../intrinsic-sizes/floats.https.html              |  78 +++++++++
 .../intrinsic-sizes/invalid-min-max.https.html     |  39 +++++
 .../intrinsic-sizes/invalid-ref.html               |  11 ++
 .../intrinsic-sizes/negative-max.https.html        |  40 +++++
 .../intrinsic-sizes/negative-min.https.html        |  40 +++++
 .../intrinsic-sizes/negative-ref.html              |  11 ++
 .../layout-child/absolute.https.html               |  43 +++++
 .../layout-child/before-after.https.html           |  48 +++++
 .../css-layout-api/layout-child/fixed.https.html   |  43 +++++
 .../css-layout-api/layout-child/float.https.html   |  43 +++++
 .../css-layout-api/layout-child/inflow.https.html  |  42 +++++
 .../layout-child/inlines-dynamic.https.html        |  39 +++++
 .../css-layout-api/layout-child/inlines.https.html |  54 ++++++
 .../layout-child/support/layout-child-worklet.js   |  26 +++
 .../css-layout-api/layout-child/text-01.https.html |  51 ++++++
 .../css-layout-api/layout-child/text-02.https.html |  44 +++++
 ...ticol-with-custom-layout-child-crash.https.html |   6 +
 .../multicol-break-before-crash.https.html         |   7 +
 .../css-layout-api/multicol-child-crash.https.html |  10 ++
 .../multicol-details-crash.https.html              |   6 +
 .../multicol-fieldset-crash.https.html             |   8 +
 .../position-fragment/htb-ltr.https.html           |  54 ++++++
 .../position-fragment/htb-rtl.https.html           |  54 ++++++
 .../css/css-layout-api/position-fragment/ref.html  |  33 ++++
 .../support/layout-position-child-worklet.js       |  20 +++
 .../position-fragment/vlr-ltr.https.html           |  54 ++++++
 .../position-fragment/vlr-rtl.https.html           |  54 ++++++
 .../position-fragment/vrl-ltr.https.html           |  54 ++++++
 .../position-fragment/vrl-rtl.https.html           |  54 ++++++
 .../css/css-layout-api/style-map-multi-ref.html    |  13 ++
 .../css/css-layout-api/style-map-multi.https.html  |  70 ++++++++
 .../tests/css/css-layout-api/style-map-ref.html    |  16 ++
 .../tests/css/css-layout-api/style-map.https.html  |  71 ++++++++
 .../tests/css/css-layout-api/supports.https.html   |  11 ++
 .../sync-layout-microtasks.https.html              |  56 ++++++
 185 files changed, 7996 insertions(+)
 create mode 100644 testing/web-platform/tests/css/css-layout-api/META.yml
 create mode 100644 testing/web-platform/tests/css/css-layout-api/at-supports-rule.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/absolute.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-absolute-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-floats-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-negative-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/flex-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/flex.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/floats.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/auto-block-size/negative.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/baseline/child-baseline.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/baseline/flex-baseline.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/baseline/no-baseline.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/baseline/orthogonal-baseline.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/box-tree-registered-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/box-tree-registered.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/box-tree-unregistered-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/box-tree-unregistered.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-htb-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-invalid.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-vrl-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-htb-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-invalid.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-vrl-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-invalid.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-invalid.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-quirks-mode.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/child-constraints/support/layout-child-sizes-worklet.js
 create mode 100644 testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-000-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-001-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/chrome-bug-1291449-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/chrome-bug-1296664-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/computed-style-layout-function.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints-data-function-failure.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints-data-sab-failure.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints-data.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-left-right-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-none.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-top-bottom.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-max.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-min.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-basis-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-basis.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-grow.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-none.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-stretch-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-grow-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-none.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-percentage-indefinite.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch-max.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-none.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch-max.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-indefinite.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-quirks-mode.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-quirky-body.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-left-right.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-top-bottom-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats-vlr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-vlr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow-column-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-grid.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage-vlr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size-quirky-body-iframe.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size.js
 create mode 100644 testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-inline-size.js
 create mode 100644 testing/web-platform/tests/css/css-layout-api/crash-multicol.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/all-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/all.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/border-htb-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/border-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/border-vlr-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/border-vlr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/border-vrl-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/border-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/padding-htb-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/padding-htb.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/padding-vlr-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/padding-vlr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/padding-vrl-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/padding-vrl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/scrollbar-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/scrollbar.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/edges/support/edges.js
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/bad-return.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/constructor-error.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/error.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/fallback-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/invalid-child.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/no-promise.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/unresolved-promise.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/bad-return.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/constructor-error.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/error.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/fallback-layout-fallback-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-child.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-fragment.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/no-promise.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fallback-layout/unresolved-promise.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fragment-data-function-failure.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fragment-data-immutable.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fragment-data-sab-failure.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/fragment-data.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/green-square-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/inline-style-layout-function.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/input-text-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/inside-multicol-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-01.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-02.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-03.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-contribution.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-min-max.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-max.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-min.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/absolute.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/before-after.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/fixed.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/float.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/inflow.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/inlines-dynamic.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/inlines.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/support/layout-child-worklet.js
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/text-01.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/layout-child/text-02.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/list-item-multicol-with-custom-layout-child-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/multicol-break-before-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/multicol-child-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/multicol-details-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/multicol-fieldset-crash.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/htb-ltr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/htb-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/support/layout-position-child-worklet.js
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-ltr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-ltr.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-rtl.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/style-map-multi-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/style-map-multi.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/style-map-ref.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/style-map.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/supports.https.html
 create mode 100644 testing/web-platform/tests/css/css-layout-api/sync-layout-microtasks.https.html

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

diff --git a/testing/web-platform/tests/css/css-layout-api/META.yml b/testing/web-platform/tests/css/css-layout-api/META.yml
new file mode 100644
index 0000000000..c85c2d4ccc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/META.yml
@@ -0,0 +1,4 @@
+spec: https://drafts.css-houdini.org/css-layout-api/
+suggested_reviewers:
+  - bfgeek
+  - tabatkins
diff --git a/testing/web-platform/tests/css/css-layout-api/at-supports-rule.https.html b/testing/web-platform/tests/css/css-layout-api/at-supports-rule.https.html
new file mode 100644
index 0000000000..0f23325ce6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/at-supports-rule.https.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#valdef-display-layout">
+<meta name="assert" content="This test checks that a layout() function works correctly with an @supports rule." />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+#test {
+  content: 'fail';
+}
+
+@supports (display: layout(foo)) {
+  #test {
+    content: 'pass';
+  }
+}
+</style>
+<div id="test"></div>
+<script>
+test(function() {
+  const element = document.getElementById('test');
+  assert_equals(getComputedStyle(element).content, '"pass"');
+});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/absolute.https.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/absolute.https.html
new file mode 100644
index 0000000000..bf81b21a4b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/absolute.https.html
@@ -0,0 +1,73 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="auto-block-size-absolute-ref.html">
+<meta name="assert" content="This test checks that the absolute positioning respects the auto-block-size." />
+
+<style>
+.test {
+  position: absolute;
+  background: red;
+}
+
+@supports (display: layout(block-size-100)) {
+  .test {
+    display: layout(block-size-100);
+    background: green;
+  }
+}
+
+.container {
+  position: relative;
+  margin: 20px 0;
+  width: 200px;
+  height: 200px;
+  border: solid 2px;
+}
+
+.width-100 {
+  width: 100px;
+  writing-mode: horizontal-tb;
+}
+
+.height-100 {
+  height: 100px;
+  writing-mode: vertical-rl;
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<!-- 100px x 100px, bottom-left -->
+<div class="container">
+  <div class="test width-100" style="bottom: 0px"></div>
+</div>
+
+<!-- 100px x 200px, left, auto-size is ignored. -->
+<div class="container">
+  <div class="test width-100" style="top: 0px; bottom: 0px"></div>
+</div>
+
+<!-- 100px x 100px, top-left -->
+<div class="container">
+  <div class="test height-100" style="left: 0px;"></div>
+</div>
+
+<!-- 100px x 100px, top, auto-size is ignored. -->
+<div class="container">
+  <div class="test height-100" style="left: 0px; right: 0px;"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('block-size-100', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: 100};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-absolute-ref.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-absolute-ref.html
new file mode 100644
index 0000000000..416d57c43f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-absolute-ref.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<style>
+.container {
+  position: relative;
+  margin: 20px 0;
+  width: 200px;
+  height: 200px;
+  border: solid 2px;
+}
+
+.result {
+  position: absolute;
+  background: green;
+}
+</style>
+<div class="container">
+  <div class="result" style="width: 100px; height: 100px; bottom: 0px;"></div>
+</div>
+<div class="container">
+  <div class="result" style="width: 100px; height: 200px;"></div>
+</div>
+<div class="container">
+  <div class="result" style="width: 100px; height: 100px;"></div>
+</div>
+<div class="container">
+  <div class="result" style="width: 200px; height: 100px;"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-floats-ref.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-floats-ref.html
new file mode 100644
index 0000000000..368d3d8693
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-floats-ref.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<style>
+.container {
+  width: 150px;
+  border: solid 2px;
+}
+
+.left {
+  float: left;
+  background: green;
+  width: 100px;
+  height: 100px;
+}
+
+.right {
+  float: right;
+  background: green;
+  width: 100px;
+  height: 100px;
+}
+</style>
+
+<div class="container">
+  <div class="left"></div>
+  <div class="right"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-negative-ref.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-negative-ref.html
new file mode 100644
index 0000000000..665b5c6620
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/auto-block-size-negative-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  border: solid 2px;
+  margin: 20px 0;
+}
+</style>
+
+<div class="result" style="width: 100px; height: 0px;"></div>
+<div class="result" style="width: 0px; height: 100px;"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/flex-ref.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/flex-ref.html
new file mode 100644
index 0000000000..e71b104c74
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/flex-ref.html
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<style>
+.container {
+  border: solid 2px;
+  display: flow-root;
+  width: 300px;
+}
+
+.child {
+  background: green;
+  border: solid 2px;
+  box-sizing: border-box;
+  float: left;
+  height: 100px;
+}
+</style>
+
+<div class="container">
+  <div class="child" style="width: 100px;"></div>
+  <div class="child" style="width: 200px;"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/flex.https.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/flex.https.html
new file mode 100644
index 0000000000..96fe3c79a9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/flex.https.html
@@ -0,0 +1,61 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="flex-ref.html">
+<meta name="assert" content="This test checks that the flex layout respects the auto-block-size." />
+
+<style>
+.flex {
+  width: 300px;
+  display: flex;
+  border: solid 2px;
+}
+
+.custom {
+  background: red;
+  box-sizing: border-box;
+  border: solid 2px;
+  height: 100px;
+  writing-mode: vertical-rl;
+}
+
+@supports (display: layout(block-size-100)) {
+  .custom-100 {
+    display: layout(block-size-100);
+    background: green;
+  }
+  .custom-50 {
+    display: layout(block-size-50);
+    background: green;
+    flex: 1;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<!-- Tests that floats using an auto block size get positioned correctly. -->
+<div class="flex">
+  <div class="custom custom-100"></div>
+  <div class="custom custom-50"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('block-size-100', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: 100};
+  }
+});
+registerLayout('block-size-50', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: 50};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/floats.https.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/floats.https.html
new file mode 100644
index 0000000000..342c57b530
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/floats.https.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="auto-block-size-floats-ref.html">
+<meta name="assert" content="This test checks that if the layout() is a float, the flow layout respects the auto-block-size." />
+
+<style>
+.test {
+  background: red;
+}
+
+@supports (display: layout(block-size-100)) {
+  .test {
+    display: layout(block-size-100);
+    background: green;
+  }
+}
+
+.container {
+  width: 150px;
+  border: solid 2px;
+}
+
+.left {
+  float: left;
+  width: 100px;
+  writing-mode: horizontal-tb;
+}
+
+.right {
+  float: right;
+  height: 100px;
+  writing-mode: vertical-rl;
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<!-- Tests that floats using an auto block size get positioned correctly. -->
+<div class="container">
+  <div class="left test"></div>
+  <div class="right test"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('block-size-100', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: 100};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow-ref.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow-ref.html
new file mode 100644
index 0000000000..73f84811b1
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow-ref.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<style>
+.result {
+  margin: 20px 0;
+  background: green;
+}
+</style>
+<div class="result" style="width: 100px; height: 100px;"></div>
+<div class="result" style="width: 100px; height: 150px;"></div>
+<div class="result" style="width: 100px; height: 50px;"></div>
+<div class="result" style="width: 100px; height: 100px;"></div>
+<div class="result" style="width: 150px; height: 100px;"></div>
+<div class="result" style="width: 50px; height: 100px;"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow.https.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow.https.html
new file mode 100644
index 0000000000..2239b27894
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/inflow.https.html
@@ -0,0 +1,63 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="inflow-ref.html">
+<meta name="assert" content="This test checks that min/max-block-size constraints are applied correctly to a layout()." />
+
+<style>
+.test {
+  margin: 20px 0;
+  background: red;
+}
+
+@supports (display: layout(block-size-100)) {
+  .test {
+    display: layout(block-size-100);
+    background: green;
+  }
+}
+
+.width-100 {
+  width: 100px;
+  writing-mode: horizontal-tb;
+}
+
+.height-100 {
+  height: 100px;
+  writing-mode: vertical-rl;
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<!-- 100px x 100px -->
+<div class="test width-100"></div>
+
+<!-- 100px x 150px -->
+<div class="test width-100" style="min-height: 150px"></div>
+
+<!-- 100px x 50px -->
+<div class="test width-100" style="max-height: 50px"></div>
+
+<!-- 100px x 100px -->
+<div class="test height-100"></div>
+
+<!-- 150px x 100px -->
+<div class="test height-100" style="min-width: 150px"></div>
+
+<!-- 50px x 100px -->
+<div class="test height-100" style="max-width: 50px"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('block-size-100', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: 100};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/auto-block-size/negative.https.html b/testing/web-platform/tests/css/css-layout-api/auto-block-size/negative.https.html
new file mode 100644
index 0000000000..d321f03002
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/auto-block-size/negative.https.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="auto-block-size-negative-ref.html">
+<meta name="assert" content="This test checks that auto-block-size is correctly clamped to zero." />
+
+<style>
+
+.test {
+  background: red;
+  border: solid 2px;
+  margin: 20px 0;
+}
+
+.width-100 {
+  width: 100px;
+  writing-mode: horizontal-tb;
+}
+
+.height-100 {
+  height: 100px;
+  writing-mode: vertical-rl;
+}
+
+@supports (display: layout(block-size-negative)) {
+  .test {
+    display: layout(block-size-negative);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test width-100"></div>
+<div class="test height-100"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('block-size-negative', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: -100};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/baseline/child-baseline.https.html b/testing/web-platform/tests/css/css-layout-api/baseline/child-baseline.https.html
new file mode 100644
index 0000000000..a871d69011
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/baseline/child-baseline.https.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-alignment">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that passing child baseline to a parent works correctly." />
+<style>
+.test {
+  background: green;
+  padding: 0 10px;
+  width: 80px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const BASELINE = 10;
+
+registerLayout('parent', class {
+  async intrinsicSizes() {}
+  async layout(children) {
+    const childFragments = await Promise.all(children.map(child => child.layoutNextFragment({})));
+
+    if (childFragments[0].baseline != BASELINE)
+      return {autoBlockSize: 0, childFragments};
+
+    return {autoBlockSize: 100, childFragments};
+  }
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {}
+  async layout(children) {
+    const childFragments = await Promise.all(children.map(child => child.layoutNextFragment({})));
+    return { childFragments, baseline: BASELINE }
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/baseline/flex-baseline.https.html b/testing/web-platform/tests/css/css-layout-api/baseline/flex-baseline.https.html
new file mode 100644
index 0000000000..ce869bee37
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/baseline/flex-baseline.https.html
@@ -0,0 +1,51 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-alignment">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that a baseline aligned element uses the baseline set in layout()."/>
+<style>
+.parent {
+  background: red;
+  display: flex;
+  padding: 0 10px;
+  width: 80px;
+  height: 100px;
+  align-items: baseline;
+}
+
+.child {
+  color: red;
+}
+
+@supports (display: layout(parent)) {
+  .parent {
+    background: green;
+  }
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="parent">
+  <div style="position: absolute; top: 50px; width: 50px; height: 25px; background: green;"></div>
+  <div></div>
+  <div class="child">text</div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('child', class {
+  async intrinsicSizes() {}
+  async layout(children) {
+    const childFragments = await Promise.all(children.map(child => child.layoutNextFragment({})));
+    return {childFragments, baseline: -50};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/baseline/no-baseline.https.html b/testing/web-platform/tests/css/css-layout-api/baseline/no-baseline.https.html
new file mode 100644
index 0000000000..0bc08552bc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/baseline/no-baseline.https.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-alignment">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that an element with no baseline is handled correctly."/>
+<style>
+.test {
+  background: green;
+  padding: 0 10px;
+  width: 80px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  async intrinsicSizes() {}
+  async layout(children) {
+    const childFragments = await Promise.all(children.map(child => child.layoutNextFragment({})));
+    if (childFragments[0].baseline)
+      return {autoBlockSize: 0, childFragments};
+    return {autoBlockSize: 100, childFragments};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/baseline/orthogonal-baseline.https.html b/testing/web-platform/tests/css/css-layout-api/baseline/orthogonal-baseline.https.html
new file mode 100644
index 0000000000..c83560b57e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/baseline/orthogonal-baseline.https.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-alignment">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that orthogonal children do not produce baselines."/>
+<style>
+.test {
+  background: green;
+  padding: 0 10px;
+  width: 80px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div style="writing-mode: vertical-rl; color: green;">text</div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('test', class {
+  async intrinsicSizes() {}
+  async layout(children) {
+    const childFragments = await Promise.all(children.map(child => child.layoutNextFragment({})));
+    if (childFragments[0].baseline)
+      return {autoBlockSize: 0, childFragments};
+    return {autoBlockSize: 100, childFragments};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/box-tree-registered-ref.html b/testing/web-platform/tests/css/css-layout-api/box-tree-registered-ref.html
new file mode 100644
index 0000000000..0ba9862015
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/box-tree-registered-ref.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<style>
+.container {
+  margin: 20px 0;
+  border: solid 2px;
+  width: 100px;
+}
+
+.pink {
+  background: hotpink;
+}
+</style>
+
+<div class="container">
+  <div class="pink" style="width: 50px; height: 100px;"></div>
+</div>
+
+<div class="container">
+  <div class="pink" style="width: 100px; height: 100px;"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/box-tree-registered.https.html b/testing/web-platform/tests/css/css-layout-api/box-tree-registered.https.html
new file mode 100644
index 0000000000..c3024f9f3a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/box-tree-registered.https.html
@@ -0,0 +1,66 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-api-box-tree">
+<link rel="match" href="box-tree-registered-ref.html">
+<meta name="assert" content="This test checks that a registered layout() has children which are blockified and new formatting contexts." />
+<style>
+@supports (display: layout(registered)) {
+  .test {
+    display: layout(registered);
+  }
+}
+
+.container {
+  margin: 20px 0;
+  border: solid 2px;
+  width: 100px;
+}
+
+.float {
+  float: left;
+  width: 50%;
+  height: 50px;
+}
+
+.pink {
+  background: hotpink;
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="container">
+  <!-- This tests that the "layout()" grandchildren floats within the don't interact with each other. -->
+  <div class="test">
+    <div class="inflow">
+      <div class="float pink"></div>
+    </div>
+    <div class="inflow">
+      <div class="float pink"></div>
+    </div>
+  </div>
+</div>
+
+<div class="container">
+  <!-- This tests that the "layout()" children margins interact as if they are new formatting contexts. -->
+  <div class="test">
+    <div class="inflow pink">
+      <div style="margin-bottom: 50px;"></div>
+    </div>
+    <div class="inflow pink">
+      <div style="margin-top: 50px;"></div>
+    </div>
+  </div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('registered', class {
+  async intrinsicSizes() {}
+  async layout() { throw Error(); }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/box-tree-unregistered-ref.html b/testing/web-platform/tests/css/css-layout-api/box-tree-unregistered-ref.html
new file mode 100644
index 0000000000..20312cff8f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/box-tree-unregistered-ref.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<style>
+.container {
+  float: left;
+  margin: 20px 0;
+  border: solid 2px;
+  width: 100px;
+}
+
+.inflow {
+  height: 40px;
+}
+</style>
+
+<div class="container">
+  <div class="inflow"></div>
+  <div class="inflow" style="background: hotpink"></div>
+  <div class="inflow"></div>
+</div>
+
+<div class="container">
+  <div class="inflow"></div>
+  <div class="inflow" style="background: hotpink"></div>
+  <div class="inflow"></div>
+</div>
+
+<div class="container">
+  <div class="inflow" style="background: hotpink"></div>
+  <div class="inflow" style="background: green"></div>
+</div>
+
+<div class="container">
+  <div style="height:100px; background:green;"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/box-tree-unregistered.https.html b/testing/web-platform/tests/css/css-layout-api/box-tree-unregistered.https.html
new file mode 100644
index 0000000000..25fe602d1c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/box-tree-unregistered.https.html
@@ -0,0 +1,62 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-api-box-tree">
+<link rel="match" href="box-tree-unregistered-ref.html">
+<meta name="assert" content="This test checks that an unregistered layout() works the same as a normal flow-root." />
+<style>
+@supports (display: layout(unregistered)) {
+  .test {
+    display: layout(unregistered);
+  }
+}
+
+.container {
+  float: left;
+  margin: 20px 0;
+  border: solid 2px;
+  width: 100px;
+}
+
+.float {
+  float: left;
+  width: 100%;
+  height: 40px;
+  background: hotpink;
+}
+
+.inflow {
+  height: 40px;
+  background: hotpink;
+}
+</style>
+
+<div class="container">
+  <!-- This tests that the "layout()" margins don't collapse with its children. -->
+  <div class="test" style="margin: 20px 0">
+    <div class="inflow" style="margin: 20px 0"></div>
+  </div>
+</div>
+
+<div class="container">
+  <!-- This tests that the "layout()" grows to fit child floats. -->
+  <div class="test">
+    <div class="float" style="margin: 40px 0"></div>
+  </div>
+</div>
+
+<div class="container">
+  <!-- This tests that a float does not intrude into "layout()" box. -->
+  <div class="float"></div>
+  <div class="test" style="width: 100%; height: 40px; background: green;"></div>
+</div>
+
+<div class="container">
+  <!-- This tests that margins in two "layout()" box children collapse as normally. -->
+  <div class="test" style="background: green;">
+    <div style="margin:100px 0;">
+      <div style="margin:100px 0;"></div>
+    </div>
+    <div style="margin:100px 0;">
+      <div style="margin:100px 0;"></div>
+    </div>
+  </div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-htb-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-htb-vrl.https.html
new file mode 100644
index 0000000000..6ec8e4062f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-htb-vrl.https.html
@@ -0,0 +1,65 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the available block-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  line-height: 0;
+
+  --available-block-size: 20;
+}
+
+.inline {
+  display: inline-block;
+  width: 8px;
+}
+
+.inline-size-10 { height: 10px; }
+.inline-size-30 { height: 30px; }
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- As the inlines don't fit within 20px, we'll end up with two lines. -->
+  <div class="child" style="--block-size-expected: 30; --inline-size-expected: 16;">
+    <span class="inline inline-size-10"></span>
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- The single inline doesn't take up the whole 20px, so will be shrink fitted. -->
+  <div class="child" style="--block-size-expected: 10; --inline-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+
+  <!-- Make sure the max-height property clamps the size. -->
+  <div class="child" style="max-height: 25px; --block-size-expected: 25; --inline-size-expected: 8;">
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- Make sure the min-height property clamps the size. -->
+  <div class="child" style="min-height: 25px; --block-size-expected: 25; --inline-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-invalid.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-invalid.https.html
new file mode 100644
index 0000000000..32a0f11a4e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-invalid.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting an invalid available block-size works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  line-height: 0;
+
+  --available-block-size: -20;
+}
+
+.inline {
+  display: inline-block;
+  width: 8px;
+}
+
+.inline-size-10 { height: 10px; }
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- The invalid available block-size should be clamped to zero. -->
+  <div class="child" style="--block-size-expected: 10; --inline-size-expected: 16;">
+    <span class="inline inline-size-10"></span>
+    <span class="inline inline-size-10"></span>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-vrl-htb.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-vrl-htb.https.html
new file mode 100644
index 0000000000..536af3b5a4
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-block-size-vrl-htb.https.html
@@ -0,0 +1,65 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the available block-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.child {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  line-height: 0;
+
+  --available-block-size: 20;
+}
+
+.inline {
+  display: inline-block;
+  height: 8px;
+}
+
+.inline-size-10 { width: 10px; }
+.inline-size-30 { width: 30px; }
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- As the inlines don't fit within 20px, we'll end up with two lines. -->
+  <div class="child" style="--block-size-expected: 30; --inline-size-expected: 16;">
+    <span class="inline inline-size-10"></span>
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- The single inline doesn't take up the whole 20px, so will be shrink fitted. -->
+  <div class="child" style="--block-size-expected: 10; --inline-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+
+  <!-- Make sure the max-width property clamps the size. -->
+  <div class="child" style="max-width: 25px; --block-size-expected: 25; --inline-size-expected: 8;">
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- Make sure the min-width property clamps the size. -->
+  <div class="child" style="min-width: 25px; --block-size-expected: 25; --inline-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-htb-htb.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-htb-htb.https.html
new file mode 100644
index 0000000000..6205c01d10
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-htb-htb.https.html
@@ -0,0 +1,65 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the available inline-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  line-height: 0;
+
+  --available-inline-size: 20;
+}
+
+.inline {
+  display: inline-block;
+  height: 8px;
+}
+
+.inline-size-10 { width: 10px; }
+.inline-size-30 { width: 30px; }
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- As the inlines don't fit within 20px, we'll end up with two lines. -->
+  <div class="child" style="--inline-size-expected: 30; --block-size-expected: 16;">
+    <span class="inline inline-size-10"></span>
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- The single inline doesn't take up the whole 20px, so will be shrink fitted. -->
+  <div class="child" style="--inline-size-expected: 10; --block-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+
+  <!-- Make sure the max-width property clamps the size. -->
+  <div class="child" style="max-width: 25px; --inline-size-expected: 25; --block-size-expected: 8;">
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- Make sure the min-width property clamps the size. -->
+  <div class="child" style="min-width: 25px; --inline-size-expected: 25; --block-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-invalid.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-invalid.https.html
new file mode 100644
index 0000000000..8bb18aaa4b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-invalid.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting an invalid available inline-size works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  line-height: 0;
+
+  --available-inline-size: -20;
+}
+
+.inline {
+  display: inline-block;
+  height: 8px;
+}
+
+.inline-size-10 { width: 10px; }
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- The invalid available inline-size should be clamped to zero. -->
+  <div class="child" style="--inline-size-expected: 10; --block-size-expected: 16;">
+    <span class="inline inline-size-10"></span>
+    <span class="inline inline-size-10"></span>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-vrl-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-vrl-vrl.https.html
new file mode 100644
index 0000000000..1b8d01f024
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-inline-size-vrl-vrl.https.html
@@ -0,0 +1,65 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the available inline-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.child {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  line-height: 0;
+
+  --available-inline-size: 20;
+}
+
+.inline {
+  display: inline-block;
+  width: 8px;
+}
+
+.inline-size-10 { height: 10px; }
+.inline-size-30 { height: 30px; }
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- As the inlines don't fit within 20px, we'll end up with two lines. -->
+  <div class="child" style="--inline-size-expected: 30; --block-size-expected: 16;">
+    <span class="inline inline-size-10"></span>
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- The single inline doesn't take up the whole 20px, so will be shrink fitted. -->
+  <div class="child" style="--inline-size-expected: 10; --block-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+
+  <!-- Make sure the max-height property clamps the size. -->
+  <div class="child" style="max-height: 25px; --inline-size-expected: 25; --block-size-expected: 8;">
+    <span class="inline inline-size-30"></span>
+  </div>
+
+  <!-- Make sure the min-height property clamps the size. -->
+  <div class="child" style="min-height: 25px; --inline-size-expected: 25; --block-size-expected: 8;">
+    <span class="inline inline-size-10"></span>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-htb.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-htb.https.html
new file mode 100644
index 0000000000..9bf4d40ad6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-htb.https.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that resolving percentages against the available size works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+
+  --available-inline-size: 50;
+  --available-block-size: 20;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed width/height shouldn't be affected by the available size. -->
+  <div class="child" style="width: 10px; height: 10px;"></div>
+
+  <!-- A percentage width/height should resolve itself against the available size. -->
+  <div class="child" style="width: 20%; height: 50%;"></div>
+
+  <!-- A percentage max-width/max-height should resolve itself against the available size. -->
+  <div class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;"></div>
+
+  <!-- A percentage min-width/min-height should resolve itself against the available size. -->
+  <div class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;"></div>
+
+  <!-- A replaced percentage width/height should resolve itself against the available size. -->
+  <img class="child" style="width: 20%; height: 50%;" />
+
+  <!-- A replaced percentage max-width/max-height should resolve itself against the available size. -->
+  <img class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;" />
+
+  <!-- A replaced percentage min-width/min-height should resolve itself against the available size. -->
+  <img class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-vrl.https.html
new file mode 100644
index 0000000000..eb104a180d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-htb-vrl.https.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that resolving percentages against the available size works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+
+  --available-inline-size: 50;
+  --available-block-size: 20;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed width/height shouldn't be affected by the available size. -->
+  <div class="child" style="width: 10px; height: 10px;"></div>
+
+  <!-- A percentage width/height should resolve itself against the available size. -->
+  <div class="child" style="width: 20%; height: 50%;"></div>
+
+  <!-- A percentage max-width/max-height should resolve itself against the available size. -->
+  <div class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;"></div>
+
+  <!-- A percentage min-width/min-height should resolve itself against the available size. -->
+  <div class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;"></div>
+
+  <!-- A replaced percentage width/height should resolve itself against the available size. -->
+  <img class="child" style="width: 20%; height: 50%;" />
+
+  <!-- A replaced percentage max-width/max-height should resolve itself against the available size. -->
+  <img class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;" />
+
+  <!-- A replaced percentage min-width/min-height should resolve itself against the available size. -->
+  <img class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-invalid.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-invalid.https.html
new file mode 100644
index 0000000000..8bd969271e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-invalid.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that resolving percentages against an invalid available size works as expected." />
+
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.child {
+  visibility: hidden;
+  width: 10px;
+  line-height: 0;
+
+  --available-inline-size: -20;
+  --available-block-size: -10;
+}
+
+.inline {
+  display: inline-block;
+  width: 10px;
+  height: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- The available inline-size gets clamped to zero, and the available block-size gets treated as indefinite. -->
+  <div class="child" style="--inline-size-expected: 0px; --block-size-expected: 10px; width: 100%; height: 100%;">
+    <div class="inline"></div>
+  </div>
+
+  <!-- For replaced elements, both axis should be resolved to 0px. -->
+  <img class="child" style="--inline-size-expected: 0px; --block-size-expected: 0px; width: 100%; height: 100%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-htb.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-htb.https.html
new file mode 100644
index 0000000000..ce8ff95dd0
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-htb.https.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that resolving percentages against the available size works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.child {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+
+  --available-inline-size: 20;
+  --available-block-size: 50;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed width/height shouldn't be affected by the available size. -->
+  <div class="child" style="width: 10px; height: 10px;"></div>
+
+  <!-- A percentage width/height should resolve itself against the available size. -->
+  <div class="child" style="width: 20%; height: 50%;"></div>
+
+  <!-- A percentage max-width/max-height should resolve itself against the available size. -->
+  <div class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;"></div>
+
+  <!-- A percentage min-width/min-height should resolve itself against the available size. -->
+  <div class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;"></div>
+
+  <!-- A replaced percentage width/height should resolve itself against the available size. -->
+  <img class="child" style="width: 20%; height: 50%;" />
+
+  <!-- A replaced percentage max-width/max-height should resolve itself against the available size. -->
+  <img class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;" />
+
+  <!-- A replaced percentage min-width/min-height should resolve itself against the available size. -->
+  <img class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-vrl.https.html
new file mode 100644
index 0000000000..71c7355b9c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/available-size-for-percentages-vrl-vrl.https.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that resolving percentages against the available size works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.child {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+
+  --available-inline-size: 20;
+  --available-block-size: 50;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed width/height shouldn't be affected by the available size. -->
+  <div class="child" style="width: 10px; height: 10px;"></div>
+
+  <!-- A percentage width/height should resolve itself against the available size. -->
+  <div class="child" style="width: 20%; height: 50%;"></div>
+
+  <!-- A percentage max-width/max-height should resolve itself against the available size. -->
+  <div class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;"></div>
+
+  <!-- A percentage min-width/min-height should resolve itself against the available size. -->
+  <div class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;"></div>
+
+  <!-- A replaced percentage width/height should resolve itself against the available size. -->
+  <img class="child" style="width: 20%; height: 50%;" />
+
+  <!-- A replaced percentage max-width/max-height should resolve itself against the available size. -->
+  <img class="child" style="width: 15px; max-width: 20%; height: 15px; max-height: 50%;" />
+
+  <!-- A replaced percentage min-width/min-height should resolve itself against the available size. -->
+  <img class="child" style="width: 5px; min-width: 20%; height: 5px; min-height: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size-vrl.https.html
new file mode 100644
index 0000000000..631c5f8281
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size-vrl.https.html
@@ -0,0 +1,60 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that fixing the block size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.htb {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-block-size: 10;
+
+  --inline-size-expected: 2;
+  --block-size-expected: 10;
+}
+
+.vrl {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-block-size: 10;
+
+  --inline-size-expected: 2;
+  --block-size-expected: 10;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="htb"></div>
+  <div class="vrl"></div>
+  <!-- min/max-width should have no effect, fixedBlockSize wins. -->
+  <div class="htb" style="max-width: 5px;"></div>
+  <div class="vrl" style="max-width: 5px;"></div>
+  <div class="htb" style="min-width: 15px;"></div>
+  <div class="vrl" style="min-width: 15px;"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size.https.html
new file mode 100644
index 0000000000..737cc4da75
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-block-size.https.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that fixing the block size of children works as expected." />
+
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.htb {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-block-size: 10;
+
+  --inline-size-expected: 3;
+  --block-size-expected: 10;
+}
+
+.vrl {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-block-size: 10;
+
+  --inline-size-expected: 3;
+  --block-size-expected: 10;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="htb"></div>
+  <div class="vrl"></div>
+  <!-- min/max-height should have no effect, fixedBlockSize wins. -->
+  <div class="htb" style="max-height: 5px;"></div>
+  <div class="vrl" style="max-height: 5px;"></div>
+  <div class="htb" style="min-height: 15px;"></div>
+  <div class="vrl" style="min-height: 15px;"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size-vrl.https.html
new file mode 100644
index 0000000000..4fc3c2e77d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size-vrl.https.html
@@ -0,0 +1,60 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedinlinesize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that fixing the inline size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.htb {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-inline-size: 10;
+
+  --inline-size-expected: 10;
+  --block-size-expected: 3;
+}
+
+.vrl {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-inline-size: 10;
+
+  --inline-size-expected: 10;
+  --block-size-expected: 3;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="htb"></div>
+  <div class="vrl"></div>
+  <!-- min/max-height should have no effect, fixedInlineSize wins. -->
+  <div class="htb" style="max-height: 5px;"></div>
+  <div class="vrl" style="max-height: 5px;"></div>
+  <div class="htb" style="min-height: 15px;"></div>
+  <div class="vrl" style="min-height: 15px;"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size.https.html
new file mode 100644
index 0000000000..0fdf4aca3a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/fixed-inline-size.https.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedinlinesize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that fixing the inline size of children works as expected." />
+
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.htb {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-inline-size: 10;
+
+  --inline-size-expected: 10;
+  --block-size-expected: 2;
+}
+
+.vrl {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  width: 3px;
+  height: 2px;
+
+  --fixed-inline-size: 10;
+
+  --inline-size-expected: 10;
+  --block-size-expected: 2;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="htb"></div>
+  <div class="vrl"></div>
+  <!-- min/max-width should have no effect, fixedInlineSize wins. -->
+  <div class="htb" style="max-width: 5px;"></div>
+  <div class="vrl" style="max-width: 5px;"></div>
+  <div class="htb" style="min-width: 15px;"></div>
+  <div class="vrl" style="min-width: 15px;"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-htb.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-htb.https.html
new file mode 100644
index 0000000000..84bda1ddf5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-htb.https.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-percentageblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the percentage block-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  width: 10px;
+
+  --percentage-block-size: 20;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed height shouldn't be affected by the percentage size. -->
+  <div class="child" style="height: 10px;"></div>
+
+  <!-- A percentage height should resolve itself against the percentageBlockSize. -->
+  <div class="child" style="height: 50%;"></div>
+
+  <!-- A percentage max-height should resolve itself against the percentageBlockSize. -->
+  <div class="child" style="height: 15px; max-height: 50%;"></div>
+
+  <!-- A percentage min-height should resolve itself against the percentageBlockSize. -->
+  <div class="child" style="height: 5px; min-height: 50%;"></div>
+
+  <!-- A replaced percentage height should resolve itself against the percentageBlockSize. -->
+  <img class="child" style="height: 50%;" />
+
+  <!-- A replaced percentage max-height should resolve itself against the percentageBlockSize. -->
+  <img class="child" style="height: 15px; max-height: 50%;" />
+
+  <!-- A replaced percentage min-height should resolve itself against the percentageBlockSize. -->
+  <img class="child" style="height: 5px; min-height: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-vrl.https.html
new file mode 100644
index 0000000000..98d285b015
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-htb-vrl.https.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-percentageinlinesize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the percentage inline-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  width: 100px;
+}
+
+.child {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  height: 10px;
+
+  --percentage-inline-size: 20;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed width shouldn't be affected by the percentage size. -->
+  <div class="child" style="width: 10px;"></div>
+
+  <!-- A percentage width should resolve itself against the percentageInlineSize. -->
+  <div class="child" style="width: 50%;"></div>
+
+  <!-- A percentage max-width should resolve itself against the percentageInlineSize. -->
+  <div class="child" style="width: 15px; max-width: 50%;"></div>
+
+  <!-- A percentage min-width should resolve itself against the percentageInlineSize. -->
+  <div class="child" style="width: 5px; min-width: 50%;"></div>
+
+  <!-- A replaced percentage width should resolve itself against the percentageInlineSize. -->
+  <img class="child" style="width: 50%;" />
+
+  <!-- A replaced percentage max-width should resolve itself against the percentageInlineSize. -->
+  <img class="child" style="width: 15px; max-width: 50%;" />
+
+  <!-- A replaced percentage min-width should resolve itself against the percentageInlineSize. -->
+  <img class="child" style="width: 5px; min-width: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-invalid.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-invalid.https.html
new file mode 100644
index 0000000000..0435535552
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-invalid.https.html
@@ -0,0 +1,47 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-percentageblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting an invalid percentage block-size of children works as expected." />
+
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.child {
+  visibility: hidden;
+  width: 10px;
+  line-height: 0;
+
+  --percentage-block-size: -10;
+}
+
+.inline {
+  display: inline-block;
+  width: 10px;
+  height: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A percentage shouldn't be resolved against an invalid percentageBlockSize. -->
+  <div class="child" style="--inline-size-expected: 10px; --block-size-expected: 10px; height: 100%;">
+    <div class="inline"></div>
+  </div>
+
+  <!-- A percentage shouldn't be resolved against an invalid percentageBlockSize. -->
+  <img class="child" style="--inline-size-expected: 10px; --block-size-expected: 0px; height: 100%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-quirks-mode.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-quirks-mode.https.html
new file mode 100644
index 0000000000..527149b67a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-quirks-mode.https.html
@@ -0,0 +1,55 @@
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-percentageblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting a percentage block-size works correctly in quirks mode." />
+
+<style>
+.container {
+  height: 200px;
+}
+
+.test {
+  background: red;
+  width: 100px;
+}
+
+.child {
+  visibility: hidden;
+  width: 10px;
+  line-height: 0;
+}
+
+.inline {
+  display: inline-block;
+  width: 10px;
+  height: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="container">
+  <div class="test">
+    <!-- TODO explain. -->
+    <div class="child" style="--percentage-block-size: 50px; --inline-size-expected: 10px; --block-size-expected: 10px;">
+      <div style="height: 20%"></div>
+    </div>
+
+    <!-- TODO explain. -->
+    <div class="child" style="--inline-size-expected: 10px; --block-size-expected: 10px;">
+      <div style="height: 100%">
+        <div class="inline"></div>
+      </div>
+    </div>
+  </div>
+</div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-htb.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-htb.https.html
new file mode 100644
index 0000000000..2d3d496fa5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-htb.https.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-percentageinlinesize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the percentage inline-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.child {
+  writing-mode: horizontal-tb;
+  visibility: hidden;
+  width: 10px;
+
+  --percentage-inline-size: 20;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed height shouldn't be affected by the percentage size. -->
+  <div class="child" style="height: 10px;"></div>
+
+  <!-- A percentage height should resolve itself against the percentageInlineSize. -->
+  <div class="child" style="height: 50%;"></div>
+
+  <!-- A percentage max-height should resolve itself against the percentageInlineSize. -->
+  <div class="child" style="height: 15px; max-height: 50%;"></div>
+
+  <!-- A percentage min-height should resolve itself against the percentageInlineSize. -->
+  <div class="child" style="height: 5px; min-height: 50%;"></div>
+
+  <!-- A replaced percentage height should resolve itself against the percentageInlineSize. -->
+  <img class="child" style="height: 50%;" />
+
+  <!-- A replaced percentage max-height should resolve itself against the percentageInlineSize. -->
+  <img class="child" style="height: 15px; max-height: 50%;" />
+
+  <!-- A replaced percentage min-height should resolve itself against the percentageInlineSize. -->
+  <img class="child" style="height: 5px; min-height: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-vrl.https.html
new file mode 100644
index 0000000000..577b98ad98
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/percentage-size-vrl-vrl.https.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-percentageblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that setting the percentage block-size of children works as expected." />
+
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  height: 100px;
+}
+
+.child {
+  writing-mode: vertical-rl;
+  visibility: hidden;
+  height: 10px;
+
+  --percentage-block-size: 20;
+
+  --inline-size-expected: 10px;
+  --block-size-expected: 10px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <!-- A fixed width shouldn't be affected by the percentage size. -->
+  <div class="child" style="width: 10px;"></div>
+
+  <!-- A percentage width should resolve itself against the percentageBlockSize. -->
+  <div class="child" style="width: 50%;"></div>
+
+  <!-- A percentage max-width should resolve itself against the percentageBlockSize. -->
+  <div class="child" style="width: 15px; max-width: 50%;"></div>
+
+  <!-- A percentage min-width should resolve itself against the percentageBlockSize. -->
+  <div class="child" style="width: 5px; min-width: 50%;"></div>
+
+  <!-- A replaced percentage width should resolve itself against the percentageBlockSize. -->
+  <img class="child" style="width: 50%;" />
+
+  <!-- A replaced percentage max-width should resolve itself against the percentageBlockSize. -->
+  <img class="child" style="width: 15px; max-width: 50%;" />
+
+  <!-- A replaced percentage min-width should resolve itself against the percentageBlockSize. -->
+  <img class="child" style="width: 5px; min-width: 50%;" />
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-sizes-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/child-constraints/support/layout-child-sizes-worklet.js b/testing/web-platform/tests/css/css-layout-api/child-constraints/support/layout-child-sizes-worklet.js
new file mode 100644
index 0000000000..5956c9a70c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/child-constraints/support/layout-child-sizes-worklet.js
@@ -0,0 +1,67 @@
+import {areArraysEqual} from '/common/arrays.js';
+
+function parseNumber(value) {
+  const num = parseInt(value.toString());
+  if (isNaN(num)) return undefined;
+  return num;
+}
+
+registerLayout('test', class {
+  static get childInputProperties() {
+    return [
+      '--available-inline-size',
+      '--available-block-size',
+      '--fixed-inline-size',
+      '--fixed-block-size',
+      '--percentage-inline-size',
+      '--percentage-block-size',
+      '--inline-size-expected',
+      '--block-size-expected'
+    ];
+  }
+
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    const childFragments = await Promise.all(children.map((child) => {
+      const childConstraints = {};
+      const availableInlineSize = parseNumber(child.styleMap.get('--available-inline-size'));
+      const availableBlockSize = parseNumber(child.styleMap.get('--available-block-size'));
+      const fixedInlineSize = parseNumber(child.styleMap.get('--fixed-inline-size'));
+      const fixedBlockSize = parseNumber(child.styleMap.get('--fixed-block-size'));
+      const percentageInlineSize = parseNumber(child.styleMap.get('--percentage-inline-size'));
+      const percentageBlockSize = parseNumber(child.styleMap.get('--percentage-block-size'));
+      return child.layoutNextFragment({
+        availableInlineSize,
+        availableBlockSize,
+        fixedInlineSize,
+        fixedBlockSize,
+        percentageInlineSize,
+        percentageBlockSize,
+      });
+    }));
+
+    const actual = childFragments.map((childFragment) => {
+      return {
+        inlineSize: childFragment.inlineSize,
+        blockSize: childFragment.blockSize,
+      };
+    });
+
+    const expected = children.map((child) => {
+      return {
+        inlineSize: parseInt(child.styleMap.get('--inline-size-expected').toString()),
+        blockSize: parseInt(child.styleMap.get('--block-size-expected').toString()),
+      };
+    });
+
+    const equalityFunc = (a, b) => {
+      return a.inlineSize == b.inlineSize && a.blockSize == b.blockSize;
+    };
+
+    if (!areArraysEqual(expected, actual, equalityFunc)) {
+      return {autoBlockSize: 0, childFragments};
+    }
+
+    return {autoBlockSize: 100, childFragments};
+  }
+});
diff --git a/testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-000-crash.https.html b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-000-crash.https.html
new file mode 100644
index 0000000000..6137eba6dc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-000-crash.https.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1287843">
+<div style="columns:2;">
+  <div style="display:layout(foo);">
+    <div>
+      <div style="display:inline-table;">
+        <div style="display:table-row;"></div>
+      </div>
+    </div>
+  </div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-001-crash.https.html b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-001-crash.https.html
new file mode 100644
index 0000000000..ac8c6ecf1a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1287843-001-crash.https.html
@@ -0,0 +1,18 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1287843">
+<div style="columns:2;">
+  <div style="display:layout(foo);">
+    <div>
+      <div style="display:inline-table;">
+        <div style="display:table-row;">
+          <div style="columns:2;">
+            <div style="display:inline-table;">
+              <div style="display:table-row;"></div>
+            </div>
+          </div>
+        </div>
+      </div>
+    </div>
+  </div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/chrome-bug-1291449-crash.https.html b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1291449-crash.https.html
new file mode 100644
index 0000000000..d13fed0179
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1291449-crash.https.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1291449">
+<div style="columns:2;">
+  <div style="display:flex;"></div>
+  <div style="display:grid;"></div>
+  <div style="display:table;"></div>
+  <div style="display:layout(foo);">
+    <div style="columns:2;"></div>
+  </div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/chrome-bug-1296664-crash.https.html b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1296664-crash.https.html
new file mode 100644
index 0000000000..9e523fb181
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/chrome-bug-1296664-crash.https.html
@@ -0,0 +1,18 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1296664">
+<div id="container">
+  <div style="position:relative; margin-top:8px;">
+    <div style="display:layout(foo); position:absolute; width:100px; height:100px;"></div>
+    <div style="column-span:all;"></div>
+  </div>
+  <span style="columns:1;">
+    <div style="display:flex;"></div>
+    <div style="display:grid;"></div>
+    <div style="display:table;"></div>
+  </span>
+</div>
+<script>
+  document.body.offsetTop;
+  container.style.columns = "1";
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/computed-style-layout-function.https.html b/testing/web-platform/tests/css/css-layout-api/computed-style-layout-function.https.html
new file mode 100644
index 0000000000..d115009c6c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/computed-style-layout-function.https.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#valdef-display-layout">
+<meta name="assert" content="This test checks that a layout() function parses correctly and serializes correctly from computed style." />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+#test1 { display: layout(test1); }
+#test2 { display: layout(); }
+#test3 { display: layout(test3, invalid); }
+#test4 { --display: layout(test4); display: var(--display); }
+</style>
+
+<div id=test1></div>
+<div id=test2></div>
+<div id=test3></div>
+<div id=test4></div>
+<div id=test5></div>
+<script>
+test(function() {
+  const test1 = document.getElementById('test1');
+  assert_equals(getComputedStyle(test1).display, 'layout(test1)');
+});
+
+test(function() {
+  // layout() should fail to parse.
+  const test2 = document.getElementById('test2');
+  assert_equals(getComputedStyle(test2).display, 'block');
+});
+
+test(function() {
+  // layout(test3, invalid) should fail to parse.
+  const test3 = document.getElementById('test3');
+  assert_equals(getComputedStyle(test3).display, 'block');
+});
+
+test(function() {
+  // Setting via a custom property should work.
+  const test4 = document.getElementById('test4');
+  assert_equals(getComputedStyle(test4).display, 'layout(test4)');
+});
+
+test(function() {
+  // Setting the inline style should reflect in the computed style.
+  const test5 = document.getElementById('test5');
+  assert_equals(getComputedStyle(test5).display, 'block');
+
+  test5.style.display = 'layout(test5)';
+  assert_equals(getComputedStyle(test5).display, 'layout(test5)');
+});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints-data-function-failure.https.html b/testing/web-platform/tests/css/css-layout-api/constraints-data-function-failure.https.html
new file mode 100644
index 0000000000..2d5e5240d7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints-data-function-failure.https.html
@@ -0,0 +1,61 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that a function can't be passed to a child layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  async intrinsicSizes() {}
+  async layout([child], edges, constraints, styleMap) {
+    let childFragment = null;
+
+    try {
+      childFragment = await child.layoutNextFragment({
+        data: { fn: function() {} }
+      });
+    } catch(e) {
+      // Success! The structured cloning algorithm should have thrown an error.
+      childFragment = await child.layoutNextFragment({});
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: 0};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints-data-sab-failure.https.html b/testing/web-platform/tests/css/css-layout-api/constraints-data-sab-failure.https.html
new file mode 100644
index 0000000000..b8df61c73d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints-data-sab-failure.https.html
@@ -0,0 +1,62 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that a SharedArrayBuffer can't be passed to a child layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  async intrinsicSizes() {}
+  async layout([child], edges, constraints, styleMap) {
+    let childFragment = null;
+
+    try {
+      childFragment = await child.layoutNextFragment({
+        // See https://github.com/whatwg/html/issues/5380 for why not `new SharedArrayBuffer()`
+        data: { sab: new WebAssembly.Memory({ shared:true, initial:1, maximum:1 }).buffer }
+      });
+    } catch(e) {
+      // Success! The structured cloning algorithm should have thrown an error.
+      childFragment = await child.layoutNextFragment({});
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {}
+  async layout() {
+    return {autoBlockSize: 0};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints-data.https.html b/testing/web-platform/tests/css/css-layout-api/constraints-data.https.html
new file mode 100644
index 0000000000..4bea061917
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints-data.https.html
@@ -0,0 +1,69 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that passing data to a child layout works correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const DATA = {
+  str: 'hello',
+  num: 42,
+  obj: {str2: 'world'},
+};
+
+registerLayout('parent', class {
+  async intrinsicSizes() {}
+  async layout([child], edges, constraints, styleMap) {
+
+    const childFragment = await child.layoutNextFragment({data: DATA});
+
+    // If the child's block-size is 100 the structured cloning worked.
+    if (childFragment.blockSize === 100) {
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    // Use JSON.stringify to make sure the structured cloning worked.
+    if (constraints.data !== DATA &&
+        JSON.stringify(constraints.data) === JSON.stringify(DATA)) {
+      return {autoBlockSize: 100};
+    }
+
+    return {autoBlockSize: 0};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-left-right-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-left-right-vrl.https.html
new file mode 100644
index 0000000000..8e177dbf33
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-left-right-vrl.https.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  position: relative;
+  width: 120px;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  position: absolute;
+  left: 0px;
+  right: 20px;
+  --expected-block-size: 100;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-none.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-none.https.html
new file mode 100644
index 0000000000..7b10f11d7d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-none.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  position: relative;
+}
+
+.test {
+  background: red;
+  position: absolute;
+  --expected-block-size: null;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-top-bottom.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-top-bottom.https.html
new file mode 100644
index 0000000000..73c29000cf
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-absolute-top-bottom.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  position: relative;
+  height: 120px;
+}
+
+.test {
+  background: red;
+  position: absolute;
+  top: 0px;
+  bottom: 20px;
+  --expected-block-size: 100;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none-vrl.https.html
new file mode 100644
index 0000000000..704b66d64a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none-vrl.https.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+.test {
+  writing-mode: vertical-lr;
+  background: red;
+  --expected-block-size: null;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none.https.html
new file mode 100644
index 0000000000..6c023f5162
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-block-none.https.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+.test {
+  background: red;
+  --expected-block-size: null;
+  width: 100px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-max.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-max.https.html
new file mode 100644
index 0000000000..8af6afde26
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-max.https.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+.test {
+  background: red;
+  --expected-block-size: 30;
+  width: 100px;
+  height: 60px;
+  max-height: 30px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-min.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-min.https.html
new file mode 100644
index 0000000000..4d76ed9785
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-min.https.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+.test {
+  background: red;
+  --expected-block-size: 70;
+  width: 100px;
+  height: 60px;
+  min-height: 70px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-vrl.https.html
new file mode 100644
index 0000000000..bbc0a2fde8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed-vrl.https.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  --expected-block-size: 100;
+  width: 100px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed.https.html
new file mode 100644
index 0000000000..45376ac295
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-fixed.https.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+.test {
+  background: red;
+  --expected-block-size: 60;
+  width: 100px;
+  height: 60px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-basis-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-basis-vrl.https.html
new file mode 100644
index 0000000000..25fc685a67
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-basis-vrl.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  --expected-block-size: 100;
+  flex-basis: 100px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-basis.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-basis.https.html
new file mode 100644
index 0000000000..8df99ee835
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-basis.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  flex-direction: column;
+  width: 100px;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 60;
+  flex-basis: 60px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-grow.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-grow.https.html
new file mode 100644
index 0000000000..da53fa86ad
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-grow.https.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  flex-direction: column;
+  width: 100px;
+  height: 60px;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 50;
+  margin-bottom: 10px;
+  flex-grow: 1;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-none.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-none.https.html
new file mode 100644
index 0000000000..1d4249e925
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-none.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  flex-direction: column;
+  width: 100px;
+}
+
+.test {
+  background: red;
+  --expected-block-size: null;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-stretch-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-stretch-vrl.https.html
new file mode 100644
index 0000000000..c71e202b0c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-column-stretch-vrl.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  flex-direction: column;
+  width: 100px;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  --expected-block-size: 90; /* flex-item should stretch to (100 - 10)px */
+  margin-left: 10px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-grow-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-grow-vrl.https.html
new file mode 100644
index 0000000000..f77b84910a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-grow-vrl.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  width: 100px;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  --expected-block-size: 100;
+  flex-grow: 1;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-none.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-none.https.html
new file mode 100644
index 0000000000..9ef11daccf
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-none.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+}
+
+.test {
+  background: red;
+  --expected-block-size: null;
+  width: 100px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-percentage-indefinite.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-percentage-indefinite.https.html
new file mode 100644
index 0000000000..111120673e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-percentage-indefinite.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+}
+
+.test {
+  background: red;
+  --expected-block-size: null; /* Percentage which we are resolving height against is indefinite. */
+  width: 100px;
+  height: 50%;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch-max.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch-max.https.html
new file mode 100644
index 0000000000..c72ed3e04d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch-max.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  height: 50px;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 30; /* flex-item should respect the max constraint */
+  max-height: 30px;
+  width: 100px;
+  margin-bottom: 10px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch.https.html
new file mode 100644
index 0000000000..cfaf38295f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-flex-stretch.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  height: 50px;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 40; /* flex-item should stretch to (50 - 10)px */
+  width: 100px;
+  margin-bottom: 10px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-none.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-none.https.html
new file mode 100644
index 0000000000..5c7381523e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-none.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: grid;
+}
+
+.test {
+  background: red;
+  --expected-block-size: null;
+  width: 100px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch-max.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch-max.https.html
new file mode 100644
index 0000000000..c738ad546d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch-max.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: grid;
+  grid: 50px / auto-flow;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 30; /* grid-item should respect the max constraint */
+  max-height: 30px;
+  width: 100px;
+  margin-bottom: 10px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch.https.html
new file mode 100644
index 0000000000..2e83f42c86
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-stretch.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: grid;
+  grid: 50px / auto-flow;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 40; /* grid-item should stretch to (50 - 10)px */
+  width: 100px;
+  margin-bottom: 10px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-vrl.https.html
new file mode 100644
index 0000000000..2c004cd916
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-grid-vrl.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  display: grid;
+  grid: auto-flow / 100px;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  --expected-block-size: 100;
+  width: 100px;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-indefinite.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-indefinite.https.html
new file mode 100644
index 0000000000..ab02cb4b03
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-indefinite.https.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+.test {
+  background: red;
+  --expected-block-size: null; /* Percentage which we are resolving height against is indefinite. */
+  width: 100px;
+  height: 50%;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-quirks-mode.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-quirks-mode.https.html
new file mode 100644
index 0000000000..285ce941b2
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-quirks-mode.https.html
@@ -0,0 +1,38 @@
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  height: 100px;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 50; /* In quirks mode we should get 100px * 50% */
+  width: 100px;
+  height: 50%;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="container">
+  <div class="test">
+    <div class="child"></div>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-vrl.https.html
new file mode 100644
index 0000000000..870bc526f8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage-vrl.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  width: 200px;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  --expected-block-size: 100;
+  width: 50%;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage.https.html
new file mode 100644
index 0000000000..335556550d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-percentage.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+body {
+  height: 200px;
+}
+
+.test {
+  background: red;
+  --expected-block-size: 100;
+  width: 100px;
+  height: 50%;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-block-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-quirky-body.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-quirky-body.https.html
new file mode 100644
index 0000000000..150426d45c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-block-size-quirky-body.https.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedblocksize">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedBlockSize is passed into the layout function correctly." />
+<style>
+iframe { border: none; width: 200px; height: 200px; }
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+<body>
+<script>
+// For this test we load a quirky iframe with the test file.
+// We can control the auto size of the iframe body element by setting the iframe height.
+const iframe = document.createElement('iframe');
+document.body.appendChild(iframe);
+iframe.addEventListener('load', () => {
+  importWorkletAndTerminateTestAfterAsyncPaint(
+    iframe.contentWindow.CSS.layoutWorklet, {url: 'constraints-fixed-block-size.js'});
+});
+iframe.src = 'support/constraints-fixed-block-size-quirky-body-iframe.html';
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-left-right.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-left-right.https.html
new file mode 100644
index 0000000000..3b81c4d13d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-left-right.https.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  position: relative;
+  width: 120px;
+}
+
+.test {
+  background: red;
+  position: absolute;
+  left: 0px;
+  right: 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-top-bottom-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-top-bottom-vrl.https.html
new file mode 100644
index 0000000000..ec94e59a17
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-absolute-top-bottom-vrl.https.html
@@ -0,0 +1,35 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  position: relative;
+  height: 120px;
+}
+
+.test {
+  background: red;
+  position: absolute;
+  left: 0px;
+  top: 0px;
+  bottom: 20px;
+  writing-mode: vertical-rl;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats-vlr.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats-vlr.https.html
new file mode 100644
index 0000000000..2758462bd3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats-vlr.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  height: 200px;
+  writing-mode: vertical-lr;
+}
+
+.float {
+  float: right;
+  width: 20px;
+  height: 100px;
+}
+
+.test {
+  background: red;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="float"></div>
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats.https.html
new file mode 100644
index 0000000000..862b9ad3b2
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-avoid-floats.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  width: 200px;
+}
+
+.float {
+  float: right;
+  width: 100px;
+  height: 20px;
+}
+
+.test {
+  background: red;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="float"></div>
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-vlr.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-vlr.https.html
new file mode 100644
index 0000000000..0c15fd12ee
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto-vlr.https.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  height: 120px;
+  writing-mode: vertical-lr;
+}
+
+.test {
+  margin-bottom: 20px;
+  background: red;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto.https.html
new file mode 100644
index 0000000000..c513c3d844
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-block-auto.https.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  width: 120px;
+}
+
+.test {
+  margin-right: 20px;
+  background: red;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed-vrl.https.html
new file mode 100644
index 0000000000..93b0c110d9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed-vrl.https.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+.test {
+  background: red;
+  height: 100px;
+  writing-mode: vertical-rl;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed.https.html
new file mode 100644
index 0000000000..d8e8062f83
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-fixed.https.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow-column-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow-column-vrl.https.html
new file mode 100644
index 0000000000..b9159965a0
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow-column-vrl.https.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  flex-flow: column;
+  width: 100px;
+  height: 100px;
+}
+
+.test {
+  background: red;
+  flex-grow: 1;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow.https.html
new file mode 100644
index 0000000000..c2532c1460
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-flex-grow.https.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  display: flex;
+  width: 100px;
+}
+
+.test {
+  background: red;
+  flex-grow: 1;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-grid.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-grid.https.html
new file mode 100644
index 0000000000..45f9f491e7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-grid.https.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  display: grid;
+  grid: auto-flow / 100px;
+}
+
+.test {
+  background: red;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage-vlr.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage-vlr.https.html
new file mode 100644
index 0000000000..2e35043777
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage-vlr.https.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  height: 200px;
+  writing-mode: vertical-lr;
+}
+
+.test {
+  background: red;
+  height: 50%;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage.https.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage.https.html
new file mode 100644
index 0000000000..c9041cf0ea
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-percentage.https.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-fixedinlinesize">
+<link rel="match" href="fixed-inline-size-ref.html">
+<meta name="assert" content="This test checks that LayoutConstraints#fixedInlineSize is passed into the layout function correctly." />
+<style>
+body {
+  width: 200px;
+}
+
+.test {
+  background: red;
+  width: 50%;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/constraints-fixed-inline-size.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-ref.html b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-ref.html
new file mode 100644
index 0000000000..e5727c0e2d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/fixed-inline-size-ref.html
@@ -0,0 +1,10 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  height: 100px;
+  width: 100px;
+}
+</style>
+
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size-quirky-body-iframe.html b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size-quirky-body-iframe.html
new file mode 100644
index 0000000000..da770e6caf
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size-quirky-body-iframe.html
@@ -0,0 +1,21 @@
+<style>
+body {
+  margin: 0;
+  --expected-block-size: 200;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  body {
+    display: layout(test);
+  }
+}
+</style>
+
+<!-- In Quirks mode, we should stretch to 100% of the inital containing block. -->
+<body>
+<div class="child"></div>
+</body>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size.js b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size.js
new file mode 100644
index 0000000000..25d73ef615
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size.js
@@ -0,0 +1,22 @@
+registerLayout('test', class {
+  static get inputProperties() {
+    return ['--expected-block-size'];
+  }
+
+  async intrinsicSizes() {}
+  async layout([child], edges, constraints, styleMap) {
+    let childFixedInlineSize = 0;
+    let childFixedBlockSize = 0;
+    if (constraints.fixedBlockSize === JSON.parse(styleMap.get('--expected-block-size'))) {
+      childFixedInlineSize = 100;
+      childFixedBlockSize = 100;
+    }
+
+    const childFragments = [await child.layoutNextFragment({
+      fixedInlineSize: childFixedInlineSize,
+      fixedBlockSize: childFixedBlockSize,
+    })];
+
+    return {childFragments};
+  }
+});
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-inline-size.js b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-inline-size.js
new file mode 100644
index 0000000000..3636f36654
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-inline-size.js
@@ -0,0 +1,9 @@
+registerLayout('test', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    if (constraints.fixedInlineSize !== 100)
+      return {autoBlockSize: 0};
+
+    return {autoBlockSize: 100};
+  }
+});
diff --git a/testing/web-platform/tests/css/css-layout-api/crash-multicol.https.html b/testing/web-platform/tests/css/css-layout-api/crash-multicol.https.html
new file mode 100644
index 0000000000..5f175100e5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/crash-multicol.https.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/">
+<meta name="assert" content="This test checks that browser doesn't crash when the layout() function is used with multicol." />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<style>
+#test {
+  display: layout('test');
+  columns: 2;
+}
+</style>
+
+<div id="test"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('test', class {
+  async intrinsicSizes() {}
+  async layout() {}
+});
+</script>
+
+<script>
+promise_test(async function() {
+  await importWorklet(CSS.layoutWorklet, document.getElementById('code').textContent);
+});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/all-ref.html b/testing/web-platform/tests/css/css-layout-api/edges/all-ref.html
new file mode 100644
index 0000000000..2a04f96e73
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/all-ref.html
@@ -0,0 +1,161 @@
+<!DOCTYPE html>
+<style>
+td { text-align: center; }
+
+.parent {
+  box-sizing: border-box;
+  width: 60px;
+  height: 60px;
+  border: solid;
+  border-width: 1px 2px 3px 4px;
+  padding: 0px 4px 8px 12px;
+  position: relative;
+}
+
+.child {
+  position: absolute;
+  width: 10px;
+  height: 10px;
+  background: green;
+}
+
+.top-left {
+  top: 0;
+  left: 12px;
+}
+
+.top-right {
+  top: 0;
+  right: 4px;
+}
+
+.bottom-left {
+  bottom: 8px;
+  left: 12px;
+}
+
+.bottom-right {
+  bottom: 8px;
+  right: 4px;
+}
+</style>
+
+<table>
+  <tr>
+    <td></td>
+    <td colspan=2>LTR</td>
+    <td colspan=2>RTL</td>
+  </tr>
+  <tr>
+    <td></td>
+    <td>Y</td>
+    <td>X</td>
+    <td>Y</td>
+    <td>X</td>
+  </tr>
+  <tr>
+    <td>HTB</td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VRL</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VLR</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+  </tr>
+</table>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/all.https.html b/testing/web-platform/tests/css/css-layout-api/edges/all.https.html
new file mode 100644
index 0000000000..e38d3666b8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/all.https.html
@@ -0,0 +1,194 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="all-ref.html">
+<meta name="assert" content="This test checks that 'all' sizes are passed to the layout correctly." />
+<style>
+td { text-align: center; }
+
+.parent {
+  box-sizing: border-box;
+  width: 60px;
+  height: 60px;
+  border: solid;
+  border-width: 1px 2px 3px 4px;
+  padding: 0px 4px 8px 12px;
+  position: relative;
+  background: red;
+}
+
+@supports (display: layout(test)) {
+  .parent {
+    display: layout(test);
+    background: initial;
+  }
+}
+
+.child {
+  width: 10px;
+  height: 10px;
+  background: green;
+}
+</style>
+<!--
+  This test works by placing four children in each corner of the layout using the edges.
+  The reference to this test uses absolute positioning to achieve the same effect.
+-->
+<table>
+  <tr>
+    <td></td>
+    <td colspan=2>LTR</td>
+    <td colspan=2>RTL</td>
+  </tr>
+  <tr>
+    <td></td>
+    <td>Y</td>
+    <td>X</td>
+    <td>Y</td>
+    <td>X</td>
+  </tr>
+  <tr>
+    <td>HTB</td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VRL</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VLR</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+  </tr>
+</table>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+<script id="code" type="text/worklet">
+registerLayout('test', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints) {
+    const topLeftFragment = await children[0].layoutNextFragment();
+    const topRightFragment = await children[1].layoutNextFragment();
+    const bottomLeftFragment = await children[2].layoutNextFragment();
+    const bottomRightFragment = await children[3].layoutNextFragment();
+
+    topLeftFragment.inlineOffset = edges.inlineStart;
+    topLeftFragment.blockOffset = edges.blockStart;
+
+    topRightFragment.inlineOffset =
+        constraints.fixedInlineSize - topRightFragment.inlineSize - edges.inlineEnd;
+    topRightFragment.blockOffset = edges.blockStart;
+
+    bottomLeftFragment.inlineOffset = edges.inlineStart;
+    bottomLeftFragment.blockOffset =
+        constraints.fixedBlockSize - bottomLeftFragment.blockSize - edges.blockEnd;
+
+    bottomRightFragment.inlineOffset =
+        constraints.fixedInlineSize - bottomRightFragment.inlineSize - edges.inlineEnd;
+    bottomRightFragment.blockOffset =
+        constraints.fixedBlockSize - bottomRightFragment.blockSize - edges.blockEnd;
+
+    return {childFragments: [
+        topLeftFragment,
+        topRightFragment,
+        bottomLeftFragment,
+        bottomRightFragment
+    ]};
+  }
+});
+</script>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/border-htb-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/border-htb-rtl.https.html
new file mode 100644
index 0000000000..cb200a7675
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/border-htb-rtl.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that border sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  direction: rtl;
+  background: red;
+  box-sizing: border-box;
+  width: 100px;
+
+  --edges-inline-start-expected: 8;
+  --edges-inline-end-expected: 20;
+  --edges-block-start-expected: 10;
+  --edges-block-end-expected: 0;
+
+  font-size: 8px;
+
+  border-color: transparent;
+  border-style: solid;
+  border-width: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/border-htb.https.html b/testing/web-platform/tests/css/css-layout-api/edges/border-htb.https.html
new file mode 100644
index 0000000000..67a3858830
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/border-htb.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that border sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  box-sizing: border-box;
+  width: 100px;
+
+  --edges-inline-start-expected: 20;
+  --edges-inline-end-expected: 8;
+  --edges-block-start-expected: 10;
+  --edges-block-end-expected: 0;
+
+  font-size: 8px;
+
+  border-color: transparent;
+  border-style: solid;
+  border-width: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/border-vlr-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/border-vlr-rtl.https.html
new file mode 100644
index 0000000000..8da84e9acc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/border-vlr-rtl.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that border sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-lr;
+  direction: rtl;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 0;
+  --edges-inline-end-expected: 10;
+  --edges-block-start-expected: 20;
+  --edges-block-end-expected: 8;
+
+  font-size: 8px;
+
+  border-color: transparent;
+  border-style: solid;
+  border-width: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/border-vlr.https.html b/testing/web-platform/tests/css/css-layout-api/edges/border-vlr.https.html
new file mode 100644
index 0000000000..b514b2f22f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/border-vlr.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that border sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-lr;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 10;
+  --edges-inline-end-expected: 0;
+  --edges-block-start-expected: 20;
+  --edges-block-end-expected: 8;
+
+  font-size: 8px;
+
+  border-color: transparent;
+  border-style: solid;
+  border-width: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/border-vrl-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/border-vrl-rtl.https.html
new file mode 100644
index 0000000000..38c8c3f046
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/border-vrl-rtl.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that border sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-rl;
+  direction: rtl;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 0;
+  --edges-inline-end-expected: 10;
+  --edges-block-start-expected: 8;
+  --edges-block-end-expected: 20;
+
+  font-size: 8px;
+
+  border-color: transparent;
+  border-style: solid;
+  border-width: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/border-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/border-vrl.https.html
new file mode 100644
index 0000000000..94c5353eb9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/border-vrl.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that border sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 10;
+  --edges-inline-end-expected: 0;
+  --edges-block-start-expected: 8;
+  --edges-block-end-expected: 20;
+
+  font-size: 8px;
+
+  border-color: transparent;
+  border-style: solid;
+  border-width: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/padding-htb-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/padding-htb-rtl.https.html
new file mode 100644
index 0000000000..dfbf2bc099
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/padding-htb-rtl.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that padding sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  direction: rtl;
+  background: red;
+  box-sizing: border-box;
+  width: 100px;
+
+  --edges-inline-start-expected: 8;
+  --edges-inline-end-expected: 20;
+  --edges-block-start-expected: 10;
+  --edges-block-end-expected: 0;
+
+  font-size: 8px;
+
+  padding: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/padding-htb.https.html b/testing/web-platform/tests/css/css-layout-api/edges/padding-htb.https.html
new file mode 100644
index 0000000000..ebad66594c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/padding-htb.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that padding sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: horizontal-tb;
+  background: red;
+  box-sizing: border-box;
+  width: 100px;
+
+  --edges-inline-start-expected: 20;
+  --edges-inline-end-expected: 8;
+  --edges-block-start-expected: 10;
+  --edges-block-end-expected: 0;
+
+  font-size: 8px;
+
+  padding: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/padding-vlr-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/padding-vlr-rtl.https.html
new file mode 100644
index 0000000000..86b0ca5425
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/padding-vlr-rtl.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that padding sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-lr;
+  direction: rtl;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 0;
+  --edges-inline-end-expected: 10;
+  --edges-block-start-expected: 20;
+  --edges-block-end-expected: 8;
+
+  font-size: 8px;
+
+  padding: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/padding-vlr.https.html b/testing/web-platform/tests/css/css-layout-api/edges/padding-vlr.https.html
new file mode 100644
index 0000000000..563884ade3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/padding-vlr.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that padding sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-lr;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 10;
+  --edges-inline-end-expected: 0;
+  --edges-block-start-expected: 20;
+  --edges-block-end-expected: 8;
+
+  font-size: 8px;
+
+  padding: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/padding-vrl-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/padding-vrl-rtl.https.html
new file mode 100644
index 0000000000..bda0a21144
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/padding-vrl-rtl.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that padding sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-rl;
+  direction: rtl;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 0;
+  --edges-inline-end-expected: 10;
+  --edges-block-start-expected: 8;
+  --edges-block-end-expected: 20;
+
+  font-size: 8px;
+
+  padding: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/padding-vrl.https.html b/testing/web-platform/tests/css/css-layout-api/edges/padding-vrl.https.html
new file mode 100644
index 0000000000..a65ecad311
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/padding-vrl.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that padding sizes are passed to the layout correctly." />
+<style>
+.test {
+  writing-mode: vertical-rl;
+  background: red;
+  box-sizing: border-box;
+  height: 100px;
+
+  --edges-inline-start-expected: 10;
+  --edges-inline-end-expected: 0;
+  --edges-block-start-expected: 8;
+  --edges-block-end-expected: 20;
+
+  font-size: 8px;
+
+  padding: 10px 1em 0 20px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    display: layout(test);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/edges.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/scrollbar-ref.html b/testing/web-platform/tests/css/css-layout-api/edges/scrollbar-ref.html
new file mode 100644
index 0000000000..cc41754c08
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/scrollbar-ref.html
@@ -0,0 +1,159 @@
+<!DOCTYPE html>
+<style>
+td { text-align: center; }
+
+.parent {
+  box-sizing: border-box;
+  width: 50px;
+  height: 50px;
+  border: solid;
+  position: relative;
+}
+
+.child {
+  position: absolute;
+  width: 10px;
+  height: 10px;
+  background: green;
+}
+
+.top-left {
+  top: 0;
+  left: 0;
+}
+
+.top-right {
+  top: 0;
+  right: 0;
+}
+
+.bottom-left {
+  bottom: 0;
+  left: 0;
+}
+
+.bottom-right {
+  bottom: 0;
+  right: 0;
+}
+</style>
+
+<table>
+  <tr>
+    <td></td>
+    <td colspan=2>LTR</td>
+    <td colspan=2>RTL</td>
+  </tr>
+  <tr>
+    <td></td>
+    <td>Y</td>
+    <td>X</td>
+    <td>Y</td>
+    <td>X</td>
+  </tr>
+  <tr>
+    <td>HTB</td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VRL</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VLR</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-y: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-x: scroll;">
+        <div class="child top-left"></div>
+        <div class="child top-right"></div>
+        <div class="child bottom-left"></div>
+        <div class="child bottom-right"></div>
+      </div>
+    </td>
+  </tr>
+</table>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/scrollbar.https.html b/testing/web-platform/tests/css/css-layout-api/edges/scrollbar.https.html
new file mode 100644
index 0000000000..76bbd4dccd
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/scrollbar.https.html
@@ -0,0 +1,192 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutedges">
+<link rel="match" href="scrollbar-ref.html">
+<meta name="assert" content="This test checks that scrollbar sizes are passed to the layout correctly." />
+<style>
+td { text-align: center; }
+
+.parent {
+  box-sizing: border-box;
+  width: 50px;
+  height: 50px;
+  border: solid;
+  position: relative;
+  background: red;
+}
+
+@supports (display: layout(test)) {
+  .parent {
+    display: layout(test);
+    background: initial;
+  }
+}
+
+.child {
+  width: 10px;
+  height: 10px;
+  background: green;
+}
+</style>
+<!--
+  This test works by placing four children in each corner of the layout using the edges.
+  The reference to this test uses absolute positioning to achieve the same effect.
+-->
+<table>
+  <tr>
+    <td></td>
+    <td colspan=2>LTR</td>
+    <td colspan=2>RTL</td>
+  </tr>
+  <tr>
+    <td></td>
+    <td>Y</td>
+    <td>X</td>
+    <td>Y</td>
+    <td>X</td>
+  </tr>
+  <tr>
+    <td>HTB</td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: ltr; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: horizontal-tb; direction: rtl; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VRL</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: ltr; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-rl; direction: rtl; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+  </tr>
+  <tr>
+    <td>VLR</td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: ltr; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-y: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+    <td>
+      <div class="parent" style="writing-mode: vertical-lr; direction: rtl; overflow-x: scroll;">
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+        <div class="child"></div>
+      </div>
+    </td>
+  </tr>
+</table>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+<script id="code" type="text/worklet">
+registerLayout('test', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints) {
+    const topLeftFragment = await children[0].layoutNextFragment();
+    const topRightFragment = await children[1].layoutNextFragment();
+    const bottomLeftFragment = await children[2].layoutNextFragment();
+    const bottomRightFragment = await children[3].layoutNextFragment();
+
+    topLeftFragment.inlineOffset = edges.inlineStart;
+    topLeftFragment.blockOffset = edges.blockStart;
+
+    topRightFragment.inlineOffset =
+        constraints.fixedInlineSize - topRightFragment.inlineSize - edges.inlineEnd;
+    topRightFragment.blockOffset = edges.blockStart;
+
+    bottomLeftFragment.inlineOffset = edges.inlineStart;
+    bottomLeftFragment.blockOffset =
+        constraints.fixedBlockSize - bottomLeftFragment.blockSize - edges.blockEnd;
+
+    bottomRightFragment.inlineOffset =
+        constraints.fixedInlineSize - bottomRightFragment.inlineSize - edges.inlineEnd;
+    bottomRightFragment.blockOffset =
+        constraints.fixedBlockSize - bottomRightFragment.blockSize - edges.blockEnd;
+
+    return {childFragments: [
+        topLeftFragment,
+        topRightFragment,
+        bottomLeftFragment,
+        bottomRightFragment
+    ]};
+  }
+});
+</script>
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/edges/support/edges.js b/testing/web-platform/tests/css/css-layout-api/edges/support/edges.js
new file mode 100644
index 0000000000..b70a42f398
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/edges/support/edges.js
@@ -0,0 +1,38 @@
+import {areArraysEqual} from '/common/arrays.js';
+
+function parseNumber(value) {
+  const num = parseInt(value.toString());
+  if (isNaN(num)) return 0;
+  return num;
+}
+
+registerLayout('test', class {
+  static get inputProperties() {
+    return [
+      '--edges-inline-start-expected',
+      '--edges-inline-end-expected',
+      '--edges-block-start-expected',
+      '--edges-block-end-expected',
+    ];
+  }
+
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    const actual = this.constructor.inputProperties.map(
+      prop => parseNumber(styleMap.get(prop))
+    );
+
+    const expected = [
+      edges.inlineStart,
+      edges.inlineEnd,
+      edges.blockStart,
+      edges.blockEnd,
+    ];
+
+    if (!areArraysEqual(expected, actual)) {
+      return {autoBlockSize: 0, childFragments: []};
+    }
+
+    return {autoBlockSize: 100, childFragment: []};
+  }
+});
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/bad-return.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/bad-return.https.html
new file mode 100644
index 0000000000..7ea0e93b86
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/bad-return.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-an-intrinsic-sizes-callback">
+<link rel="match" href="fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with the intrinsicSizes function returning a bad value will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: min-content;
+}
+
+.float {
+  float: left;
+  height: 100px;
+  width: 50%;
+}
+
+.fc {
+  display: flow-root;
+  height: 100px;
+}
+
+@supports (display: layout(bad-return)) {
+  .test {
+    display: layout(bad-return);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="float"></div>
+  <div class="fc"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('bad-return', class {
+  async intrinsicSizes() { return 42; }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/constructor-error.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/constructor-error.https.html
new file mode 100644
index 0000000000..2078a49cda
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/constructor-error.https.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#get-a-layout-class-instance">
+<link rel="match" href="fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with a throwing constructor will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: min-content;
+}
+
+.float {
+  float: left;
+  width: 50%;
+  height: 100px;
+}
+
+.fc {
+  display: flow-root;
+  height: 100px;
+}
+
+@supports (display: layout(throwing-ctor)) {
+  .test {
+    display: layout(throwing-ctor);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="float"></div>
+  <div class="fc"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('throwing-ctor', class {
+  constructor() { throw Error('fail!'); }
+  async intrinsicSizes() {}
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/error.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/error.https.html
new file mode 100644
index 0000000000..3a6665d8aa
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/error.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-an-intrinsic-sizes-callback">
+<link rel="match" href="fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with a throwing intrinsicSizes function will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: min-content;
+}
+
+.float {
+  float: left;
+  height: 100px;
+  width: 50%;
+}
+
+.fc {
+  display: flow-root;
+  height: 100px;
+}
+
+@supports (display: layout(throwing-intrinsic-sizes)) {
+  .test {
+    display: layout(throwing-intrinsic-sizes);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="float"></div>
+  <div class="fc"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('throwing-intrinsic-sizes', class {
+  async intrinsicSizes() { throw Error('fail!'); }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/fallback-ref.html b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/fallback-ref.html
new file mode 100644
index 0000000000..6808e14eb6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/fallback-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  border: solid 2px;
+  height: 100px;
+  width: min-content;
+}
+</style>
+
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/invalid-child.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/invalid-child.https.html
new file mode 100644
index 0000000000..c21d1577e4
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/invalid-child.https.html
@@ -0,0 +1,84 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-an-intrinsic-sizes-callback">
+<link rel="match" href="fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class performing intrinsicSizes on an invalid child will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: min-content;
+}
+
+.test > div {
+  height: 100px;
+}
+
+@supports (display: layout(bad-child-layout)) {
+  .test {
+    display: layout(bad-child-layout);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('bad-child-layout', class {
+  static get inputProperties() { return ['--fail']; }
+
+  async intrinsicSizes(children, _, styleMap) {
+    if (styleMap.get('--fail').toString() !== 'true') {
+      this.child = children[0];
+    }
+
+    // Try to perform layout on the child. If its invalid (we skipped the if
+    // statement above) we should fallback to block layout.
+    const childIntrinsicSize = await this.child.intrinsicSizes();
+
+    return { maxContentSize: 100, minContentSize: 100 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+function raf() {
+  return new Promise((resolve) => {
+    requestAnimationFrame(() => {
+      resolve();
+    });
+  });
+}
+
+(async function() {
+  if (typeof CSS.layoutWorklet === 'undefined') {
+    takeScreenshot();
+    return;
+  }
+
+  await importWorklet(CSS.layoutWorklet, document.getElementById('code').textContent);
+
+  // Ensure that all instances have a child to perform an invalid intrinsic size upon.
+  const test = document.getElementsByClassName('test')[0];
+  for (let i = 0; i < 100; i++) {
+    test.innerHTML = '<div><div>';
+    await raf();
+  }
+
+  // The next layout should mean that we will fallback to block.
+  test.innerHTML = '<div></div>';
+  test.style.setProperty('--fail', 'true');
+
+  // Finish up the test.
+  await raf();
+  await raf();
+  takeScreenshot();
+})();
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/no-promise.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/no-promise.https.html
new file mode 100644
index 0000000000..c5ec721b54
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/no-promise.https.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-an-intrinsic-sizes-callback">
+<link rel="match" href="fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with an intrinsicSizes function that doesn't return a promise will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: min-content;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(no-promise)) {
+  .test {
+    display: layout(no-promise);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('no-promise', class {
+  intrinsicSizes() { return { minContentSize: 100 }; }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/unresolved-promise.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/unresolved-promise.https.html
new file mode 100644
index 0000000000..f56e331a5a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-intrinsic-sizes/unresolved-promise.https.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-an-intrinsic-sizes-callback">
+<link rel="match" href="fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with an intrinsicSizes function that doesn't return a promise will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: min-content;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(unresolved-promise)) {
+  .test {
+    display: layout(unresolved-promise);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('unresolved-promise', class {
+  intrinsicSizes() { return new Promise(() => { /* never resolves */ }); }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/bad-return.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/bad-return.https.html
new file mode 100644
index 0000000000..3e671537c9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/bad-return.https.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-a-layout-callback">
+<link rel="match" href="fallback-layout-fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with the layout function returning a bad value will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: 100px;
+}
+
+.float {
+  float: left;
+  width: 50%;
+  height: 100px;
+}
+
+.fc {
+  display: flow-root;
+  height: 100px;
+}
+
+@supports (display: layout(bad-return)) {
+  .test {
+    display: layout(bad-return);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<!-- This tests that when the "layout()" layout function returns a bad value, it will fallback to block layout. -->
+<div class="test">
+  <div class="float"></div>
+  <div class="fc"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('bad-return', class {
+  async intrinsicSizes() {}
+  async layout() { return 42; }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/constructor-error.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/constructor-error.https.html
new file mode 100644
index 0000000000..9ce791ab5b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/constructor-error.https.html
@@ -0,0 +1,51 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#get-a-layout-class-instance">
+<link rel="match" href="fallback-layout-fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with a throwing constructor will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: 100px;
+}
+
+.float {
+  float: left;
+  width: 50%;
+  height: 100px;
+}
+
+.fc {
+  display: flow-root;
+  height: 100px;
+}
+
+@supports (display: layout(throwing-ctor)) {
+  .test {
+    display: layout(throwing-ctor);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<!-- This tests that when the "layout()" constructor fails, it will fallback to block layout. -->
+<div class="test">
+  <div class="float"></div>
+  <div class="fc"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('throwing-ctor', class {
+  constructor() { throw Error('fail!'); }
+  async intrinsicSizes() {}
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/error.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/error.https.html
new file mode 100644
index 0000000000..0631193e1f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/error.https.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-a-layout-callback">
+<link rel="match" href="fallback-layout-fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with a throwing layout function will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: 100px;
+}
+
+.float {
+  float: left;
+  width: 50%;
+  height: 100px;
+}
+
+.fc {
+  display: flow-root;
+  height: 100px;
+}
+
+@supports (display: layout(throwing-layout)) {
+  .test {
+    display: layout(throwing-layout);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<!-- This tests that when the "layout()" layout function fails, it will fallback to block layout. -->
+<div class="test">
+  <div class="float"></div>
+  <div class="fc"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('throwing-layout', class {
+  async intrinsicSizes() {}
+  async layout() { throw Error('fail!'); }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/fallback-layout-fallback-ref.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/fallback-layout-fallback-ref.html
new file mode 100644
index 0000000000..63bb91e90c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/fallback-layout-fallback-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  border: solid 2px;
+  height: 100px;
+  width: 100px;
+}
+</style>
+
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-child.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-child.https.html
new file mode 100644
index 0000000000..fb48ac7602
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-child.https.html
@@ -0,0 +1,84 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-a-layout-callback">
+<link rel="match" href="fallback-layout-fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class performing layout on an invalid child will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: 100px;
+}
+
+.test > div {
+  height: 100px;
+}
+
+@supports (display: layout(bad-child-layout)) {
+  .test {
+    display: layout(bad-child-layout);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('bad-child-layout', class {
+  static get inputProperties() { return ['--fail']; }
+
+  async intrinsicSizes() {}
+  async layout(children, _, __, styleMap) {
+    if (styleMap.get('--fail').toString() !== 'true') {
+      this.child = children[0];
+    }
+
+    // Try to perform layout on the child. If its invalid (we skipped the if
+    // statement above) we should fallback to block layout.
+    const fragment = await this.child.layoutNextFragment({});
+
+    return {autoBlockSize: 0, childFragments: [fragment]};
+  }
+});
+</script>
+
+<script>
+function raf() {
+  return new Promise((resolve) => {
+    requestAnimationFrame(() => {
+      resolve();
+    });
+  });
+}
+
+(async function() {
+  if (typeof CSS.layoutWorklet === 'undefined') {
+    takeScreenshot();
+    return;
+  }
+
+  await importWorklet(CSS.layoutWorklet, document.getElementById('code').textContent);
+
+  // Ensure that all instances have a child to perform an invalid layout upon.
+  const test = document.getElementsByClassName('test')[0];
+  for (let i = 0; i < 100; i++) {
+    test.innerHTML = '<div><div>';
+    await raf();
+  }
+
+  // The next layout should mean that we will fallback to block.
+  test.innerHTML = '<div></div>';
+  test.style.setProperty('--fail', 'true');
+
+  // Finish up the test.
+  await raf();
+  await raf();
+  takeScreenshot();
+})();
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-fragment.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-fragment.https.html
new file mode 100644
index 0000000000..d954f44ba3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/invalid-fragment.https.html
@@ -0,0 +1,82 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-a-layout-callback">
+<link rel="match" href="fallback-layout-fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class returning an invalid fragment will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: 100px;
+}
+
+.test > div {
+  height: 100px;
+}
+
+@supports (display: layout(bad-request)) {
+  .test {
+    display: layout(bad-request);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('bad-request', class {
+  static get inputProperties() { return ['--fail']; }
+
+  async intrinsicSizes() {}
+  async layout(children, _, __, styleMap) {
+    if (styleMap.get('--fail').toString() !== 'true') {
+      this.fragment = await children[0].layoutNextFragment({});
+    }
+
+    // Return, if the fragment is invalid (we skipped the if statement above)
+    // we should fallback to block layout.
+    return {autoBlockSize: 0, childFragments: [this.fragment]};
+  }
+});
+</script>
+
+<script>
+function raf() {
+  return new Promise((resolve) => {
+    requestAnimationFrame(() => {
+      resolve();
+    });
+  });
+}
+
+(async function() {
+  if (typeof CSS.layoutWorklet === 'undefined') {
+    takeScreenshot();
+    return;
+  }
+
+  await importWorklet(CSS.layoutWorklet, document.getElementById('code').textContent);
+
+  // Ensure that all instances have a child to perform an invalid layout upon.
+  const test = document.getElementsByClassName('test')[0];
+  for (let i = 0; i < 100; i++) {
+    test.innerHTML = '<div><div>';
+    await raf();
+  }
+
+  // The next layout should mean that we will fallback to block.
+  test.innerHTML = '<div></div>';
+  test.style.setProperty('--fail', 'true');
+
+  // Finish up the test.
+  await raf();
+  await raf();
+  takeScreenshot();
+})();
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/no-promise.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/no-promise.https.html
new file mode 100644
index 0000000000..00670f7bbe
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/no-promise.https.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-a-layout-callback">
+<link rel="match" href="fallback-layout-fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with a layout function that doesn't return a promise will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: 100px;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(no-promise)) {
+  .test {
+    display: layout(no-promise);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('no-promise', class {
+  async intrinsicSizes() {}
+  layout() { return {autoBlockSize: 50}; }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fallback-layout/unresolved-promise.https.html b/testing/web-platform/tests/css/css-layout-api/fallback-layout/unresolved-promise.https.html
new file mode 100644
index 0000000000..72ce549acf
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fallback-layout/unresolved-promise.https.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-a-layout-callback">
+<link rel="match" href="fallback-layout-fallback-ref.html">
+<meta name="assert" content="This test checks that a layout() class with a layout function that doesn't return a promise will fallback to block layout." />
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  width: 100px;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(unresolved-promise)) {
+  .test {
+    display: layout(unresolved-promise);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('unresolved-promise', class {
+  async intrinsicSizes() {}
+  layout() { return new Promise(() => { /* never resolves */ }); }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fragment-data-function-failure.https.html b/testing/web-platform/tests/css/css-layout-api/fragment-data-function-failure.https.html
new file mode 100644
index 0000000000..8496967be0
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data-function-failure.https.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that passing something that can't be serialized to the parent triggers a fallback to block layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(fallback-fn)) {
+  .test {
+    display: layout(fallback-fn);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('fallback-fn', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    const childFragments = await children.map(child => child.layoutNextFragment());
+    return {autoBlockSize: 0, childFragments, data: {fn: function() {}}};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fragment-data-immutable.https.html b/testing/web-platform/tests/css/css-layout-api/fragment-data-immutable.https.html
new file mode 100644
index 0000000000..e33a6ff0ee
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data-immutable.https.html
@@ -0,0 +1,69 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutFragment#data is immutable between child layout passes." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  async intrinsicSizes() {}
+  async layout([child], edges, constraints, styleMap) {
+
+    const childFragment10 = await child.layoutNextFragment({fixedInlineSize: 10});
+
+    // First layout data should be "10".
+    if (childFragment10.data.size !== 10) {
+      return {autoBlockSize: 0, childFragments: [childFragment10]};
+    }
+
+    const childFragment20 = await child.layoutNextFragment({fixedInlineSize: 20});
+
+    // Second layout data should be "20".
+    if (childFragment20.data.size !== 20) {
+      return {autoBlockSize: 0, childFragments: [childFragment10]};
+    }
+
+    // First layout data should still be "10".
+    if (childFragment10.data.size !== 10) {
+      return {autoBlockSize: 0, childFragments: [childFragment10]};
+    }
+
+    return {autoBlockSize: 100, childFragments: [childFragment20]};
+  }
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    return {autoBlockSize: 10, data: {size: constraints.fixedInlineSize}};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fragment-data-sab-failure.https.html b/testing/web-platform/tests/css/css-layout-api/fragment-data-sab-failure.https.html
new file mode 100644
index 0000000000..681de5b543
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data-sab-failure.https.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that a SharedArrayBuffer can't be passed to the parent layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(fallback-sab)) {
+  .test {
+    display: layout(fallback-sab);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('fallback-sab', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    const childFragments = await Promise.all(children.map(child => child.layoutNextFragment()));
+
+    // See https://github.com/whatwg/html/issues/5380 for why not `new SharedArrayBuffer()`
+    return {autoBlockSize: 0, childFragments, data: {sab: new WebAssembly.Memory({ shared:true, initial:1, maximum:1 }).buffer }};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/fragment-data.https.html b/testing/web-platform/tests/css/css-layout-api/fragment-data.https.html
new file mode 100644
index 0000000000..ffc360ab35
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data.https.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that passing data to a parent layout works correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const DATA = {
+  str: 'hello',
+  num: 42,
+  obj: {str2: 'world'},
+};
+
+registerLayout('parent', class {
+  async intrinsicSizes() {}
+  async layout([child], edges, constraints, styleMap) {
+
+    const childFragment = await child.layoutNextFragment();
+
+    // Use JSON.stringify to make sure the structured cloning worked.
+    if (childFragment.data !== DATA &&
+        JSON.stringify(childFragment.data) === JSON.stringify(DATA)) {
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    return {autoBlockSize: 10, data: DATA};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/green-square-ref.html b/testing/web-platform/tests/css/css-layout-api/green-square-ref.html
new file mode 100644
index 0000000000..a28e43be4a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/green-square-ref.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  height: 100px;
+  width: 100px;
+}
+</style>
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/inline-style-layout-function.https.html b/testing/web-platform/tests/css/css-layout-api/inline-style-layout-function.https.html
new file mode 100644
index 0000000000..f11405b8e8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/inline-style-layout-function.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#valdef-display-layout">
+<meta name="assert" content="This test checks that a layout() function is parses and serializes correctly from inline style." />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<div id=test1></div>
+<div id=test2></div>
+<div id=test3></div>
+<script>
+test(function() {
+  const test1 = document.getElementById('test1');
+  assert_equals(test1.style.display, '');
+
+  test1.style.display = 'layout(test1)';
+  assert_equals(test1.style.display, 'layout(test1)');
+});
+
+test(function() {
+  const test2 = document.getElementById('test2');
+  assert_equals(test2.style.display, '');
+
+  // layout() should fail to parse.
+  test2.style.display = 'layout()';
+  assert_equals(test2.style.display, '');
+});
+
+test(function() {
+  const test3 = document.getElementById('test3');
+  assert_equals(test3.style.display, '');
+
+  // layout(test3, invalid) should fail to parse.
+  test3.style.display = 'layout(test3, invalid)';
+  assert_equals(test3.style.display, '');
+});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/input-text-crash.https.html b/testing/web-platform/tests/css/css-layout-api/input-text-crash.https.html
new file mode 100644
index 0000000000..2d32609040
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/input-text-crash.https.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1181687">
+<input style="display:layout(success);">
diff --git a/testing/web-platform/tests/css/css-layout-api/inside-multicol-crash.https.html b/testing/web-platform/tests/css/css-layout-api/inside-multicol-crash.https.html
new file mode 100644
index 0000000000..61223bfcdc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/inside-multicol-crash.https.html
@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1127112#c7">
+<div style="columns:16;">
+  <div style="display:layout(child-layout);">
+    <div></div>
+  </div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute-ref.html
new file mode 100644
index 0000000000..2f23f68b60
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute-ref.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<style>
+.test {
+  background: green;
+  position: absolute;
+}
+
+.container-1 {
+  border: solid 2px;
+  position: relative;
+  height: 200px;
+  width: 300px;
+}
+
+.container-2 {
+  border: solid 2px;
+  position: relative;
+  height: 25px;
+  width: 25px;
+}
+
+.horizontal {
+  writing-mode: horizontal-tb;
+}
+
+.vertical {
+  writing-mode: vertical-rl;
+}
+
+</style>
+
+<div class="container-1">
+  <div class="horizontal test" style="height: 100px; width: 100px; bottom: 0px;"></div>
+  <div class="vertical test" style="height: 100px; width: 100px; right: 0px;"></div>
+</div>
+
+<div class="container-2">
+  <div class="horizontal test" style="height: 25px; width: 50px; bottom: 0px;"></div>
+</div>
+
+<div class="container-2">
+  <div class="vertical test" style="height: 50px; width: 25px; right: 0px;"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute.https.html
new file mode 100644
index 0000000000..be654ecbc7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/absolute.https.html
@@ -0,0 +1,73 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="absolute-ref.html">
+<meta name="assert" content="This test checks that the absolute positioning respects the intrinsicSizes function." />
+
+<style>
+.test {
+  background: red;
+  position: absolute;
+}
+
+@supports (display: layout(test-layout)) {
+  .test {
+    display: layout(test-layout);
+    background: green;
+  }
+}
+
+.container-1 {
+  border: solid 2px;
+  position: relative;
+  height: 200px;
+  width: 300px;
+}
+
+.container-2 {
+  border: solid 2px;
+  position: relative;
+  height: 25px;
+  width: 25px;
+}
+
+.horizontal {
+  writing-mode: horizontal-tb;
+}
+
+.vertical {
+  writing-mode: vertical-rl;
+}
+
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+
+<div class="container-1">
+  <div class="horizontal test" style="height: 100px; bottom: 0px;"></div>
+  <div class="vertical test" style="width: 100px; right: 0px;"></div>
+</div>
+
+<div class="container-2">
+  <div class="horizontal test" style="height: 25px; bottom: 0px;"></div>
+</div>
+
+<div class="container-2">
+  <div class="vertical test" style="width: 25px; right: 0px;"></div>
+</div>
+
+
+<script id="code" type="text/worklet">
+registerLayout('test-layout', class {
+  async intrinsicSizes() {
+    return { maxContentSize: 100, minContentSize: 50 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-01.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-01.https.html
new file mode 100644
index 0000000000..058428601d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-01.https.html
@@ -0,0 +1,47 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#intrinsic-sizes">
+<link rel="match" href="child-replaced-percentage-ref.html">
+<meta name="assert" content="This test checks that intrinsicSizes is calculated correctly for a replaced child element with a percentage height." />
+<style>
+.test {
+  background: red;
+  height: 50px;
+  width: min-content;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <canvas width=100 height=100 style="height: 100%;"></canvas>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  async intrinsicSizes(children, edges, styleMap) {
+    const childrenSizes = await Promise.all(children.map((child) => {
+        return child.intrinsicSizes();
+    }));
+
+    if (childrenSizes[0].minContentSize == 50 &&
+        childrenSizes[0].maxContentSize == 50) {
+      return { maxContentSize: 100, minContentSize: 50 };
+    }
+    return { maxContentSize: 0, minContentSize: 0 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-02.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-02.https.html
new file mode 100644
index 0000000000..42e8207b5b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-02.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#intrinsic-sizes">
+<link rel="match" href="child-replaced-percentage-ref.html">
+<meta name="assert" content="This test checks that intrinsicSizes is calculated correctly for a replaced child element with a percentage height." />
+<style>
+.test {
+  background: red;
+  height: 50px;
+  width: min-content;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div style="height: 50%">
+    <canvas width=100 height=100 style="height: 100%;"></canvas>
+  </div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  async intrinsicSizes(children, edges, styleMap) {
+    const childrenSizes = await Promise.all(children.map((child) => {
+        return child.intrinsicSizes();
+    }));
+
+    if (childrenSizes[0].minContentSize == 25 &&
+        childrenSizes[0].maxContentSize == 25) {
+      return { maxContentSize: 100, minContentSize: 50 };
+    }
+    return { maxContentSize: 0, minContentSize: 0 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-ref.html
new file mode 100644
index 0000000000..762740ffee
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-replaced-percentage-ref.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  height: 50px;
+  width: 50px;
+}
+</style>
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01-ref.html
new file mode 100644
index 0000000000..3702d72080
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01-ref.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  height: 100px;
+  width: 50px;
+}
+</style>
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01.https.html
new file mode 100644
index 0000000000..c0a00e28bb
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-01.https.html
@@ -0,0 +1,61 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#intrinsic-sizes">
+<link rel="match" href="child-size-01-ref.html">
+<meta name="assert" content="This test checks that passing child intrinsicSizes to a parent works correctly." />
+<style>
+.test {
+  background: red;
+  height: 100px;
+  width: min-content;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const MAX_CONTENT_SIZE = 100;
+const MIN_CONTENT_SIZE = 50;
+
+registerLayout('parent', class {
+  async intrinsicSizes(children, edges, styleMap) {
+    const childrenSizes = await Promise.all(children.map((child) => {
+        return child.intrinsicSizes();
+    }));
+
+    if (childrenSizes[0].minContentSize == MIN_CONTENT_SIZE &&
+        childrenSizes[0].maxContentSize == MAX_CONTENT_SIZE) {
+      return { maxContentSize: MAX_CONTENT_SIZE, minContentSize: MIN_CONTENT_SIZE };
+    }
+    return { maxContentSize: 0, minContentSize: 0 };
+  }
+  async layout() {}
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {
+    return { maxContentSize: MAX_CONTENT_SIZE, minContentSize: MIN_CONTENT_SIZE };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02-ref.html
new file mode 100644
index 0000000000..76c885e182
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02-ref.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  height: 100px;
+  width: 75px;
+}
+</style>
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02.https.html
new file mode 100644
index 0000000000..030ecfccdc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-02.https.html
@@ -0,0 +1,67 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#intrinsic-sizes">
+<link rel="match" href="child-size-02-ref.html">
+<meta name="assert" content="This test checks that setting a child's intrinsicSizes does not override its min-width." />
+<style>
+.test {
+  background: red;
+  height: 100px;
+  width: min-content;
+}
+
+.child {
+  min-width: 75px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const MAX_CONTENT_SIZE = 100;
+const MIN_CONTENT_SIZE = 50;
+
+registerLayout('parent', class {
+  static get childInputProperties() { return ['min-width']; }
+
+  async intrinsicSizes(children, edges, styleMap) {
+    const childrenSizes = await Promise.all(children.map((child) => {
+        return child.intrinsicSizes();
+    }));
+
+    if (childrenSizes[0].minContentSize == child.styleMap.get('min-width').value &&
+        childrenSizes[0].maxContentSize == MAX_CONTENT_SIZE) {
+      return { maxContentSize: MAX_CONTENT_SIZE, minContentSize: MIN_CONTENT_SIZE };
+    }
+    return { maxContentSize: 0, minContentSize: 0 };
+  }
+  async layout() {}
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {
+    return { maxContentSize: MAX_CONTENT_SIZE, minContentSize: MIN_CONTENT_SIZE };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-03.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-03.https.html
new file mode 100644
index 0000000000..8b540cf959
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-03.https.html
@@ -0,0 +1,67 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#intrinsic-sizes">
+<link rel="match" href="child-size-02-ref.html">
+<meta name="assert" content="This test checks that setting a child's intrinsicSizes does not override its max-width." />
+<style>
+.test {
+  background: red;
+  height: 100px;
+  width: max-content;
+}
+
+.child {
+  max-width: 75px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const MAX_CONTENT_SIZE = 100;
+const MIN_CONTENT_SIZE = 50;
+
+registerLayout('parent', class {
+  static get childInputProperties() { return ['max-width']; }
+
+  async intrinsicSizes(children, edges, styleMap) {
+    const childrenSizes = await Promise.all(children.map((child) => {
+        return child.intrinsicSizes();
+    }));
+
+    if (childrenSizes[0].minContentSize == MIN_CONTENT_SIZE &&
+        childrenSizes[0].maxContentSize == child.styleMap.get('max-width').value) {
+      return { maxContentSize: MAX_CONTENT_SIZE, minContentSize: MIN_CONTENT_SIZE };
+    }
+    return { maxContentSize: 0, minContentSize: 0 };
+  }
+  async layout() {}
+});
+
+registerLayout('child', class {
+  async intrinsicSizes() {
+    return { maxContentSize: MAX_CONTENT_SIZE, minContentSize: MIN_CONTENT_SIZE };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-contribution.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-contribution.https.html
new file mode 100644
index 0000000000..e21085c9f6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/child-size-contribution.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#intrinsic-sizes">
+<link rel="match" href="child-size-01-ref.html">
+<meta name="assert" content="This test checks that the child intrinsicSizes returns the min/max size contributions." />
+<style>
+.test {
+  background: red;
+  height: 100px;
+  width: min-content;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div style="width: 100px;">
+    <div style="width: 200px;"></div>
+  </div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  async intrinsicSizes(children, edges, styleMap) {
+    const childrenSizes = await Promise.all(children.map((child) => {
+        return child.intrinsicSizes();
+    }));
+
+    if (childrenSizes[0].minContentSize == 100 &&
+        childrenSizes[0].maxContentSize == 100) {
+      return { maxContentSize: 100, minContentSize: 50 };
+    }
+    return { maxContentSize: 0, minContentSize: 0 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size-ref.html
new file mode 100644
index 0000000000..6bccf070a9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<style>
+.test {
+  background: green;
+  border: solid 2px;
+  height: 100px;
+}
+</style>
+
+<div class="test" style="width: 96px"></div>
+<div class="test" style="width: 46px"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size.https.html
new file mode 100644
index 0000000000..ed87dd5065
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/content-size.https.html
@@ -0,0 +1,48 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="content-size-ref.html">
+<meta name="assert" content="This test checks that the min-content and max-content sizes respect the values returned from the intrinsicSizes function." />
+
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  height: 100px;
+}
+
+.test-1 {
+  width: max-content;
+}
+
+.test-2 {
+  width: min-content;
+}
+
+@supports (display: layout(test-layout)) {
+  .test {
+    display: layout(test-layout);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+
+<div class="test test-1"></div>
+<div class="test test-2"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('test-layout', class {
+  async intrinsicSizes() {
+    return { maxContentSize: 100, minContentSize: 50 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats-ref.html
new file mode 100644
index 0000000000..11cdb055eb
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats-ref.html
@@ -0,0 +1,47 @@
+<!DOCTYPE html>
+<style>
+.test {
+  background: green;
+  border: solid 2px;
+}
+
+.container-1 {
+  height: 100px;
+  width: 300px;
+}
+
+.container-2 {
+  height: 50px;
+  width: 50px;
+}
+
+.container-3 {
+  height: 80px;
+  width: 80px;
+}
+
+.left {
+  float: left;
+  writing-mode: horizontal-tb;
+}
+
+.right {
+  float: right;
+  writing-mode: vertical-rl;
+}
+</style>
+
+<div class="container-1">
+  <div class="left test" style="height: 100px; width: 96px;"></div>
+  <div class="right test" style="height: 96px; width: 100px;"></div>
+</div>
+
+<div class="container-2">
+  <div class="left test" style="height: 25px; width: 46px;"></div>
+  <div class="right test" style="height: 46px; width: 25px;"></div>
+</div>
+
+<div class="container-3">
+  <div class="left test" style="height: 25px; width: 76px;"></div>
+  <div class="right test" style="height: 76px; width: 25px;"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats.https.html
new file mode 100644
index 0000000000..5079b193fe
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/floats.https.html
@@ -0,0 +1,78 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="floats-ref.html">
+<meta name="assert" content="This test checks that if the layout() is a float, the flow layout respects the intrinsicSizes function." />
+
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+}
+
+@supports (display: layout(test-layout)) {
+  .test {
+    display: layout(test-layout);
+    background: green;
+  }
+}
+
+.container-1 {
+  height: 100px;
+  width: 300px;
+}
+
+.container-2 {
+  height: 50px;
+  width: 50px;
+}
+
+.container-3 {
+  height: 80px;
+  width: 80px;
+}
+
+.left {
+  float: left;
+  writing-mode: horizontal-tb;
+}
+
+.right {
+  float: right;
+  writing-mode: vertical-rl;
+}
+
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+
+<div class="container-1">
+  <div class="left test" style="height: 100px"></div>
+  <div class="right test" style="width: 100px"></div>
+</div>
+
+<div class="container-2">
+  <div class="left test" style="height: 25px"></div>
+  <div class="right test" style="width: 25px"></div>
+</div>
+
+<div class="container-3">
+  <div class="left test" style="height: 25px"></div>
+  <div class="right test" style="width: 25px"></div>
+</div>
+
+
+<script id="code" type="text/worklet">
+registerLayout('test-layout', class {
+  async intrinsicSizes() {
+    return { maxContentSize: 100, minContentSize: 50 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-min-max.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-min-max.https.html
new file mode 100644
index 0000000000..2b2d8ca76e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-min-max.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="invalid-ref.html">
+<meta name="assert" content="This test checks that min-content-size greater than max-content-size is correctly clamped to max-content-size." />
+
+<style>
+.test {
+  background: red;
+  border: solid 2px;
+  height: 100px;
+  width: max-content;
+}
+
+@supports (display: layout(invalid-sizes)) {
+  .test {
+    display: layout(invalid-sizes);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('invalid-sizes', class {
+  async intrinsicSizes() {
+    return { maxContentSize: 10, minContentSize: 200 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-ref.html
new file mode 100644
index 0000000000..4159356047
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/invalid-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  border: solid 2px;
+  height: 100px;
+  width: 6px;
+}
+</style>
+
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-max.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-max.https.html
new file mode 100644
index 0000000000..578803767e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-max.https.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="negative-ref.html">
+<meta name="assert" content="This test checks that max-content-size is correctly clamped to zero." />
+
+<style>
+
+.test {
+  background: red;
+  border: solid 2px;
+  height: 100px;
+  width: max-content;
+}
+
+@supports (display: layout(max-content-size-negative)) {
+  .test {
+    display: layout(max-content-size-negative);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('max-content-size-negative', class {
+  async intrinsicSizes() {
+    return { maxContentSize: -100 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-min.https.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-min.https.html
new file mode 100644
index 0000000000..c614a2fb12
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-min.https.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#interaction-sizing">
+<link rel="match" href="negative-ref.html">
+<meta name="assert" content="This test checks that min-content-size is correctly clamped to zero." />
+
+<style>
+
+.test {
+  background: red;
+  border: solid 2px;
+  height: 100px;
+  width: min-content;
+}
+
+@supports (display: layout(min-content-size-negative)) {
+  .test {
+    display: layout(min-content-size-negative);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('min-content-size-negative', class {
+  async intrinsicSizes() {
+    return { minContentSize: -100 };
+  }
+  async layout() {}
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-ref.html b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-ref.html
new file mode 100644
index 0000000000..6808e14eb6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/intrinsic-sizes/negative-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  border: solid 2px;
+  height: 100px;
+  width: min-content;
+}
+</style>
+
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/absolute.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/absolute.https.html
new file mode 100644
index 0000000000..7c47e38e7a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/absolute.https.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that absolute children don't appear in the children array." />
+
+<style>
+.test {
+  --child-expected: ["2"];
+
+  background: red;
+  width: 100px;
+}
+
+.absolute {
+  position: absolute;
+  visibility: hidden;
+  --child: 1;
+}
+
+.inflow {
+  visibility: hidden;
+  --child: 2;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="absolute"></div>
+  <div class="inflow"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/before-after.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/before-after.https.html
new file mode 100644
index 0000000000..373392ddfd
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/before-after.https.html
@@ -0,0 +1,48 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that boxes created by ::before/::after appear as children." />
+
+<style>
+.test {
+  --child-expected: ["1", "2", "3"];
+
+  background: red;
+  width: 100px;
+}
+
+.test::before {
+  visibility: hidden;
+  content: 'before';
+  --child: 1;
+}
+
+.inflow {
+  visibility: hidden;
+  --child: 2;
+}
+
+.test::after {
+  visibility: hidden;
+  content: 'after';
+  --child: 3;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="inflow"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/fixed.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/fixed.https.html
new file mode 100644
index 0000000000..95d8852b7d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/fixed.https.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that fixed children don't appear in the children array." />
+
+<style>
+.test {
+  --child-expected: ["2"];
+
+  background: red;
+  width: 100px;
+}
+
+.fixed {
+  position: fixed;
+  visibility: hidden;
+  --child: 1;
+}
+
+.inflow {
+  visibility: hidden;
+  --child: 2;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="fixed"></div>
+  <div class="inflow"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/float.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/float.https.html
new file mode 100644
index 0000000000..e8db261774
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/float.https.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that float children appear in the children array." />
+
+<style>
+.test {
+  --child-expected: ["1", "2"];
+
+  background: red;
+  width: 100px;
+}
+
+.float {
+  float: right;
+  visibility: hidden;
+  --child: 1;
+}
+
+.inflow {
+  visibility: hidden;
+  --child: 2;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="float"></div>
+  <div class="inflow"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/inflow.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/inflow.https.html
new file mode 100644
index 0000000000..b43f1a7fac
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/inflow.https.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that regular inflow children appear as children." />
+
+<style>
+.test {
+  --child-expected: ["1", "2"];
+
+  background: red;
+  width: 100px;
+}
+
+.inflow-1 {
+  visibility: hidden;
+  --child: 1;
+}
+
+.inflow-2 {
+  visibility: hidden;
+  --child: 2;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="inflow-1"></div>
+  <div class="inflow-2"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/inlines-dynamic.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/inlines-dynamic.https.html
new file mode 100644
index 0000000000..50052087f4
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/inlines-dynamic.https.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS Layout API: Dynamic blockification of inline children</title>
+<link rel="author" href="mailto:obrufau@igalia.com" title="Oriol Brufau">
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children" title="4.1. Layout Children">
+<meta name="assert" content="This test checks that inline children are correctly blockified or unblockified when the display of the parent changes dynamically." />
+
+<style>
+#wrapper {
+  display: layout(foo);
+}
+#test {
+  display: inline;
+}
+</style>
+
+<div id="wrapper">
+  <div id="test">Lorem ipsum</div>
+</div>
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+<script>
+promise_test(async function() {
+  await importWorklet(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+
+  const wrapper = document.getElementById("wrapper");
+  const test = document.getElementById("test");
+
+  assert_equals(getComputedStyle(test).display, "block", "The child should have been blockified by the custom layout");
+
+  wrapper.style.display = "block";
+  assert_equals(getComputedStyle(test).display, "inline", "The child should no longer be blockified in block layout");
+
+  wrapper.style.display = "";
+  assert_equals(getComputedStyle(test).display, "block", "The child should have been blockified again");
+});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/inlines.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/inlines.https.html
new file mode 100644
index 0000000000..dac7e0b26b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/inlines.https.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that inline children are correctly blockified or wrapped in anonymous boxes." />
+
+<style>
+/* We have a wrapper in this test to ensure that any text that is positioned
+ * slightly outside the "test" box doesn't affect the rendering.
+ * This wrapper has a 10px inline padding which does the trick. */
+.wrapper {
+  background: green;
+  padding: 0 10px;
+  width: 80px;
+}
+
+.test {
+  --child-expected: ["1", "default", "3", "4", "5"];
+
+  background: red;
+  color: green;
+  width: 80px;
+  --child: default;
+}
+
+.inflow {
+  visibility: hidden;
+  --child: 3;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="wrapper">
+  <div class="test">
+    <span style="--child: 1;">Text,</span> more text
+    <div class="inflow"></div>
+    <span style="--child: 4;">Text,
+       <div>block!</div>
+    </span>
+    <span style="--child: 5;">Other text</span>
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/support/layout-child-worklet.js b/testing/web-platform/tests/css/css-layout-api/layout-child/support/layout-child-worklet.js
new file mode 100644
index 0000000000..70d1b7e457
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/support/layout-child-worklet.js
@@ -0,0 +1,26 @@
+import {areArraysEqual} from '/common/arrays.js';
+
+registerLayout('test', class {
+  static get inputProperties() {
+    return [ '--child-expected'];
+  }
+
+  static get childInputProperties() {
+    return [ '--child' ];
+  }
+
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    const expected = JSON.parse(styleMap.get('--child-expected').toString());
+    const actual = children.map((child) => {
+      return child.styleMap.get('--child').toString().trim();
+    });
+
+    const childFragments = await Promise.all(children.map(child => child.layoutNextFragment({})));
+
+    if (!areArraysEqual(expected, actual))
+      return {autoBlockSize: 0, childFragments};
+
+    return {autoBlockSize: 100, childFragments};
+  }
+});
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/text-01.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/text-01.https.html
new file mode 100644
index 0000000000..090034fc18
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/text-01.https.html
@@ -0,0 +1,51 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that text children are correctly blockified." />
+
+<style>
+/* We have a wrapper in this test to ensure that any text that is positioned
+ * slightly outside the "test" box doesn't affect the rendering.
+ * This wrapper has a 10px inline padding which does the trick. */
+.wrapper {
+  background: green;
+  padding: 0 10px;
+  width: 80px;
+}
+
+.test {
+  --child-expected: ["default", "2", "default"];
+
+  background: red;
+  color: green;
+  width: 80px;
+  --child: default;
+}
+
+.inflow {
+  visibility: hidden;
+  --child: 2;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="wrapper">
+  <div class="test">
+    Text text text
+    <div class="inflow"></div>
+    Text text text
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/layout-child/text-02.https.html b/testing/web-platform/tests/css/css-layout-api/layout-child/text-02.https.html
new file mode 100644
index 0000000000..0df7498668
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child/text-02.https.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
+<link rel="match" href="../green-square-ref.html">
+<meta name="assert" content="This test checks that text children are correctly blockified when no other children exist." />
+
+<style>
+/* We have a wrapper in this test to ensure that any text that is positioned
+ * slightly outside the "test" box doesn't affect the rendering.
+ * This wrapper has a 10px inline padding which does the trick. */
+.wrapper {
+  background: green;
+  padding: 0 10px;
+  width: 80px;
+}
+
+.test {
+  --child-expected: ["default"];
+
+  background: red;
+  color: green;
+  width: 80px;
+  --child: default;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="wrapper">
+  <div class="test">
+    Text text text
+  </div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/list-item-multicol-with-custom-layout-child-crash.https.html b/testing/web-platform/tests/css/css-layout-api/list-item-multicol-with-custom-layout-child-crash.https.html
new file mode 100644
index 0000000000..be91346c54
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/list-item-multicol-with-custom-layout-child-crash.https.html
@@ -0,0 +1,6 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1127112#c8">
+<div style="display:list-item; columns:2;">
+  <div style="display:layout(child-layout);">Do not crash.</div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/multicol-break-before-crash.https.html b/testing/web-platform/tests/css/css-layout-api/multicol-break-before-crash.https.html
new file mode 100644
index 0000000000..c2a33802e5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/multicol-break-before-crash.https.html
@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://www.w3.org/TR/css-layout-api-1/#breaking-and-fragmentation">
+<div style="columns:3; column-fill:auto; height:100px;">
+  <div style="height:1px;"></div>
+  <div style="display:layout(child-layout); break-before:column; height:200px;"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/multicol-child-crash.https.html b/testing/web-platform/tests/css/css-layout-api/multicol-child-crash.https.html
new file mode 100644
index 0000000000..e16532f772
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/multicol-child-crash.https.html
@@ -0,0 +1,10 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1127112#c9">
+<div style="columns:2;">
+  <div style="display:layout(child-layout);">
+    line1<br>
+    line2<br>
+    line3<br>
+  </div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/multicol-details-crash.https.html b/testing/web-platform/tests/css/css-layout-api/multicol-details-crash.https.html
new file mode 100644
index 0000000000..8c45190cec
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/multicol-details-crash.https.html
@@ -0,0 +1,6 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1219874">
+<div style="columns:2;">
+  <details style="display:layout(foo); columns:2;"></details>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/multicol-fieldset-crash.https.html b/testing/web-platform/tests/css/css-layout-api/multicol-fieldset-crash.https.html
new file mode 100644
index 0000000000..a267a6d42b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/multicol-fieldset-crash.https.html
@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1221885">
+<div style="columns:2;">
+  <fieldset style="display:layout(child-layout); columns:2;">
+    BOO!
+  </fieldset>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/htb-ltr.https.html b/testing/web-platform/tests/css/css-layout-api/position-fragment/htb-ltr.https.html
new file mode 100644
index 0000000000..f60109ca57
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/htb-ltr.https.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layoutfragment">
+<link rel="match" href="ref.html">
+<meta name="assert" content="This test checks that child fragments get positioned correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+  height: 100px;
+}
+
+.test {
+  writing-mode: horizontal-tb;
+  direction: ltr;
+}
+
+.child-1 {
+  background: rebeccapurple;
+  width: 10px;
+  height: 20px;
+
+  --inline-offset: 5;
+  --block-offset: 25;
+}
+
+.child-2 {
+  writing-mode: vertical-rl;
+  background: rebeccapurple;
+  width: 15px;
+  height: 25px;
+
+  --inline-offset: 50;
+  --block-offset: 60;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child-1"></div>
+  <div class="child-2"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-position-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/htb-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/position-fragment/htb-rtl.https.html
new file mode 100644
index 0000000000..980a3cdc25
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/htb-rtl.https.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layoutfragment">
+<link rel="match" href="ref.html">
+<meta name="assert" content="This test checks that child fragments get positioned correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+  height: 100px;
+}
+
+.test {
+  writing-mode: horizontal-tb;
+  direction: rtl;
+}
+
+.child-1 {
+  background: rebeccapurple;
+  width: 10px;
+  height: 20px;
+
+  --inline-offset: 85;
+  --block-offset: 25;
+}
+
+.child-2 {
+  writing-mode: vertical-rl;
+  background: rebeccapurple;
+  width: 15px;
+  height: 25px;
+
+  --inline-offset: 35;
+  --block-offset: 60;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child-1"></div>
+  <div class="child-2"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-position-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/ref.html b/testing/web-platform/tests/css/css-layout-api/position-fragment/ref.html
new file mode 100644
index 0000000000..4ce0a6e39d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/ref.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<style>
+.result {
+  position: relative;
+  background: green;
+  width: 100px;
+  height: 100px;
+}
+
+.result-child-1 {
+  background: rebeccapurple;
+  width: 10px;
+  height: 20px;
+
+  position: absolute;
+  top: 25px;
+  left: 5px;
+}
+
+.result-child-2 {
+  background: rebeccapurple;
+  width: 15px;
+  height: 25px;
+
+  position: absolute;
+  top: 60px;
+  left: 50px;
+}
+</style>
+<div class="result">
+  <div class="result-child-1"></div>
+  <div class="result-child-2"></div>
+</div>
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/support/layout-position-child-worklet.js b/testing/web-platform/tests/css/css-layout-api/position-fragment/support/layout-position-child-worklet.js
new file mode 100644
index 0000000000..7d5c494952
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/support/layout-position-child-worklet.js
@@ -0,0 +1,20 @@
+registerLayout('test', class {
+  static get childInputProperties() {
+    return [
+      '--inline-offset',
+      '--block-offset',
+    ];
+  }
+
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    const childFragments = await Promise.all(children.map((child) => child.layoutNextFragment({})));
+
+    for (let i = 0; i < children.length; i++) {
+      childFragments[i].inlineOffset = parseInt(children[i].styleMap.get('--inline-offset').toString());
+      childFragments[i].blockOffset = parseInt(children[i].styleMap.get('--block-offset').toString());
+    }
+
+    return {autoBlockSize: 0, childFragments};
+  }
+});
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-ltr.https.html b/testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-ltr.https.html
new file mode 100644
index 0000000000..d75a4af639
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-ltr.https.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layoutfragment">
+<link rel="match" href="ref.html">
+<meta name="assert" content="This test checks that child fragments get positioned correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+  height: 100px;
+}
+
+.test {
+  writing-mode: vertical-lr;
+  direction: ltr;
+}
+
+.child-1 {
+  background: rebeccapurple;
+  width: 10px;
+  height: 20px;
+
+  --inline-offset: 25;
+  --block-offset: 5;
+}
+
+.child-2 {
+  writing-mode: vertical-rl;
+  background: rebeccapurple;
+  width: 15px;
+  height: 25px;
+
+  --inline-offset: 60;
+  --block-offset: 50;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child-1"></div>
+  <div class="child-2"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-position-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-rtl.https.html
new file mode 100644
index 0000000000..a8ef6c699c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/vlr-rtl.https.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layoutfragment">
+<link rel="match" href="ref.html">
+<meta name="assert" content="This test checks that child fragments get positioned correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+  height: 100px;
+}
+
+.test {
+  writing-mode: vertical-lr;
+  direction: rtl;
+}
+
+.child-1 {
+  background: rebeccapurple;
+  width: 10px;
+  height: 20px;
+
+  --inline-offset: 55;
+  --block-offset: 5;
+}
+
+.child-2 {
+  writing-mode: vertical-rl;
+  background: rebeccapurple;
+  width: 15px;
+  height: 25px;
+
+  --inline-offset: 15;
+  --block-offset: 50;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child-1"></div>
+  <div class="child-2"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-position-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-ltr.https.html b/testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-ltr.https.html
new file mode 100644
index 0000000000..21c9ce54d6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-ltr.https.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layoutfragment">
+<link rel="match" href="ref.html">
+<meta name="assert" content="This test checks that child fragments get positioned correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+  height: 100px;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  direction: ltr;
+}
+
+.child-1 {
+  background: rebeccapurple;
+  width: 10px;
+  height: 20px;
+
+  --inline-offset: 25;
+  --block-offset: 85;
+}
+
+.child-2 {
+  writing-mode: vertical-rl;
+  background: rebeccapurple;
+  width: 15px;
+  height: 25px;
+
+  --inline-offset: 60;
+  --block-offset: 35;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child-1"></div>
+  <div class="child-2"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-position-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-rtl.https.html b/testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-rtl.https.html
new file mode 100644
index 0000000000..777e725da9
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/position-fragment/vrl-rtl.https.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layoutfragment">
+<link rel="match" href="ref.html">
+<meta name="assert" content="This test checks that child fragments get positioned correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+  height: 100px;
+}
+
+.test {
+  writing-mode: vertical-rl;
+  direction: rtl;
+}
+
+.child-1 {
+  background: rebeccapurple;
+  width: 10px;
+  height: 20px;
+
+  --inline-offset: 55;
+  --block-offset: 85;
+}
+
+.child-2 {
+  writing-mode: vertical-rl;
+  background: rebeccapurple;
+  width: 15px;
+  height: 25px;
+
+  --inline-offset: 15;
+  --block-offset: 35;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child-1"></div>
+  <div class="child-2"></div>
+</div>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, {url: 'support/layout-position-child-worklet.js'});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/style-map-multi-ref.html b/testing/web-platform/tests/css/css-layout-api/style-map-multi-ref.html
new file mode 100644
index 0000000000..fc54068418
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/style-map-multi-ref.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  margin: 10px;
+  margin-left: 2px;
+
+  width: 100px;
+  height: 100px;
+}
+</style>
+
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/style-map-multi.https.html b/testing/web-platform/tests/css/css-layout-api/style-map-multi.https.html
new file mode 100644
index 0000000000..fe6017b80c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/style-map-multi.https.html
@@ -0,0 +1,70 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-invalidation">
+<link rel="match" href="style-map-multi-ref.html">
+<meta name="assert" content="This test checks that properties are correctly given to the layout function." />
+
+<style>
+.test {
+  background: red;
+  margin: 10px;
+  width: 100px;
+
+  /* Properties under test. */
+  --foo:bar;
+  margin-left: 2px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+    display: layout(test);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test"></div>
+
+<script id="code" type="text/worklet">
+registerLayout('test', class {
+  static get inputProperties() {
+    return [ '--bar', '--foo', 'empty-cells', 'margin-left'];
+  }
+
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    const expected = [
+      {property: '--bar', value: '[CSSUnparsedValue=]'},
+      {property: '--foo', value: '[CSSUnparsedValue=bar]'},
+      {property: 'empty-cells', value: '[CSSKeywordValue=show]'},
+      {property: 'margin-left', value: '[CSSUnitValue=2px]'},
+    ];
+
+    const actual = Array.from(styleMap.keys()).sort().map((property) => {
+      const valueObject = styleMap.get(property);
+      const value = '[' + valueObject.constructor.name + '=' + valueObject.toString() + ']';
+      return {property, value};
+    });
+
+    if (expected.length != actual.length)
+      return {autoBlockSize: 0};
+
+    for (let i = 0; i < expected.length; i++) {
+      if (expected[i].property != actual[i].property)
+        return {autoBlockSize: 0};
+
+      if (expected[i].value != actual[i].value)
+        return {autoBlockSize: 0};
+    }
+
+    return {autoBlockSize: 100};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/style-map-ref.html b/testing/web-platform/tests/css/css-layout-api/style-map-ref.html
new file mode 100644
index 0000000000..c24e3949ee
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/style-map-ref.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  margin: 10px;
+  margin-left: 2px;
+
+  width: 100px;
+  height: 100px;
+}
+</style>
+
+<div class="result"></div>
+<div class="result"></div>
+<div class="result"></div>
+<div class="result"></div>
diff --git a/testing/web-platform/tests/css/css-layout-api/style-map.https.html b/testing/web-platform/tests/css/css-layout-api/style-map.https.html
new file mode 100644
index 0000000000..52032d9c67
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/style-map.https.html
@@ -0,0 +1,71 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-invalidation">
+<link rel="match" href="style-map-ref.html">
+<meta name="assert" content="This test checks that properties are correctly given to the layout function." />
+
+<style>
+.test {
+  background: red;
+  margin: 10px;
+  width: 100px;
+
+  /* Properties under test. */
+  --foo:bar;
+  margin-left: 2px;
+}
+
+@supports (display: layout(test)) {
+  .test {
+    background: green;
+  }
+
+  .test-0 { display: layout(test-0); }
+  .test-1 { display: layout(test-1); }
+  .test-2 { display: layout(test-2); }
+  .test-3 { display: layout(test-3); }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test test-0"></div>
+<div class="test test-1"></div>
+<div class="test test-2"></div>
+<div class="test test-3"></div>
+
+<script>
+const tmpl = (test, idx) => {
+  return `
+    registerLayout('test-${idx}', class {
+      static get inputProperties() { return ['${test.property}']; }
+
+      async intrinsicSizes() {}
+      async layout(children, edges, constraints, styleMap) {
+        const value = styleMap.get('${test.property}');
+        const result = '[' + value.constructor.name + '=' + value.toString() + ']';
+        if (result != '${test.expected}')
+          return {autoBlockSize: 0};
+
+        const size = Array.from(styleMap.keys()).length;
+        if (size != 1)
+          return {autoBlockSize: 0};
+
+        return {autoBlockSize: 100};
+      }
+    });
+  `;
+}
+
+const tests = [
+  {property: '--bar', expected: '[CSSUnparsedValue=]'},
+  {property: '--foo', expected: '[CSSUnparsedValue=bar]'},
+  {property: 'empty-cells', expected: '[CSSKeywordValue=show]'},
+  {property: 'margin-left', expected: '[CSSUnitValue=2px]'},
+];
+
+const workletSource = tests.map(tmpl).join('\n');
+
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, workletSource);
+</script>
+</html>
diff --git a/testing/web-platform/tests/css/css-layout-api/supports.https.html b/testing/web-platform/tests/css/css-layout-api/supports.https.html
new file mode 100644
index 0000000000..e269b29214
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/supports.https.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#valdef-display-layout">
+<meta name="assert" content="This test checks that a layout() function works correctly with CSS.supports()." />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<script>
+test(function() {
+  assert_true(CSS.supports('display', 'layout(foo)'));
+});
+</script>
diff --git a/testing/web-platform/tests/css/css-layout-api/sync-layout-microtasks.https.html b/testing/web-platform/tests/css/css-layout-api/sync-layout-microtasks.https.html
new file mode 100644
index 0000000000..84457c0c9d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/sync-layout-microtasks.https.html
@@ -0,0 +1,56 @@
+<!DOCTYPE html>
+<html>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#invoke-a-layout-callback">
+<meta name="assert" content="This test checks running the microtask queue for a layout() class won't run the main world's microtask queue." />
+<style>
+#test {
+  display: layout(child-layout);
+  width: 100px;
+}
+
+#test > div {
+  height: 100px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div id="test">
+  <div></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('child-layout', class {
+  async intrinsicSizes() {}
+  async layout([child]) {
+    const fragment = await child.layoutNextFragment();
+    return {autoBlockSize: 50, childFragments: [fragment]};
+  }
+});
+</script>
+
+<script>
+promise_test(async t => {
+  if (typeof CSS.layoutWorklet === 'undefined') {
+    throw Error('CSS Layout API not supported.');
+  }
+
+  await importWorklet(CSS.layoutWorklet, document.getElementById('code').textContent);
+
+  let resolved = false;
+  let p = Promise.resolve().then(() => {
+    resolved = true;
+  });
+  assert_false(resolved);
+
+  // Running the layout-worklet's microtask queue shouldn't trigger the main
+  // world's microtask queue.
+  assert_equals(document.getElementById('test').clientHeight, 50);
+  assert_false(resolved);
+
+  await p;
+  assert_true(resolved);
+});
+</script>
+</html>
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