From 43a97878ce14b72f0981164f87f2e35e14151312 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sun, 7 Apr 2024 11:22:09 +0200
Subject: Adding upstream version 110.0.1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../tests/css/css-properties-values-api/META.yml   |   4 +
 .../css-properties-values-api/animate-invalid.html |  20 +
 ...custom-property-animation-angle-comma-list.html |  59 ++
 ...custom-property-animation-angle-space-list.html |  59 ++
 .../animation/custom-property-animation-angle.html |  57 ++
 ...custom-property-animation-color-comma-list.html |  59 ++
 ...custom-property-animation-color-space-list.html |  59 ++
 .../animation/custom-property-animation-color.html |  57 ++
 .../custom-property-animation-custom-ident.html    |  13 +
 .../animation/custom-property-animation-image.html |  13 +
 ...mation-inherited-used-by-standard-property.html |  29 +
 ...stom-property-animation-integer-comma-list.html |  59 ++
 ...stom-property-animation-integer-space-list.html |  59 ++
 .../custom-property-animation-integer.html         |  57 ++
 ...ustom-property-animation-length-comma-list.html |  59 ++
 ...rty-animation-length-percentage-comma-list.html |  59 ++
 ...rty-animation-length-percentage-space-list.html |  59 ++
 ...ustom-property-animation-length-percentage.html |  57 ++
 ...ustom-property-animation-length-space-list.html |  59 ++
 .../custom-property-animation-length.html          |  57 ++
 ...on-non-inherited-used-by-standard-property.html |  27 +
 ...ustom-property-animation-number-comma-list.html |  59 ++
 ...ustom-property-animation-number-space-list.html |  59 ++
 .../custom-property-animation-number.html          |  57 ++
 ...m-property-animation-percentage-comma-list.html |  59 ++
 ...m-property-animation-percentage-space-list.html |  59 ++
 .../custom-property-animation-percentage.html      |  57 ++
 ...m-property-animation-resolution-comma-list.html |  59 ++
 ...m-property-animation-resolution-space-list.html |  59 ++
 .../custom-property-animation-resolution.html      |  57 ++
 .../custom-property-animation-time-comma-list.html |  59 ++
 .../custom-property-animation-time-space-list.html |  59 ++
 .../animation/custom-property-animation-time.html  |  57 ++
 ...stom-property-animation-transform-function.html |  57 ++
 ...y-animation-transform-list-multiple-values.html |  66 ++
 ...rty-animation-transform-list-single-values.html |  57 ++
 .../animation/custom-property-animation-url.html   |  13 +
 .../custom-property-transition-angle.html          |  16 +
 .../custom-property-transition-color.html          |  16 +
 .../custom-property-transition-custom-ident.html   |  15 +
 .../custom-property-transition-image.html          |  15 +
 ...sition-inherited-used-by-standard-property.html |  31 +
 .../custom-property-transition-integer.html        |  16 +
 ...stom-property-transition-length-percentage.html |  16 +
 .../custom-property-transition-length.html         |  16 +
 ...custom-property-transition-mismatched-list.html | 153 ++++
 ...on-non-inherited-used-by-standard-property.html |  28 +
 .../custom-property-transition-number.html         |  16 +
 .../custom-property-transition-percentage.html     |  16 +
 .../custom-property-transition-resolution.html     |  16 +
 .../animation/custom-property-transition-time.html |  16 +
 ...tom-property-transition-transform-function.html |  16 +
 .../custom-property-transition-transform-list.html |  16 +
 .../animation/custom-property-transition-url.html  |  15 +
 .../at-property-animation.html                     | 298 +++++++
 .../at-property-cssom.html                         | 197 ++++
 .../at-property-non-matching-media-crash.html      |  19 +
 .../at-property-shadow.html                        |  46 +
 .../at-property-stylesheets.html                   | 106 +++
 .../at-property-typedom.html                       |  64 ++
 .../at-property-viewport-units-dynamic.html        |  41 +
 .../at-property-viewport-units.html                |  91 ++
 .../css/css-properties-values-api/at-property.html | 224 +++++
 .../conditional-rules.html                         |  43 +
 .../crashtests/initial-in-audio-crash.html         |  13 +
 .../determine-registration.html                    | 232 +++++
 .../font-size-animation.html                       |  44 +
 .../css/css-properties-values-api/idlharness.html  |  16 +
 .../property-cascade.html                          |  63 ++
 .../register-property-syntax-parsing.html          | 222 +++++
 .../register-property.html                         |  75 ++
 .../registered-properties-inheritance.html         |  89 ++
 .../registered-property-change-style-001.html      |  50 ++
 .../registered-property-computation.html           | 175 ++++
 .../registered-property-crosstalk.html             |  43 +
 .../registered-property-cssom.html                 |  91 ++
 .../registered-property-initial.html               |  71 ++
 .../registered-property-revert.html                |  94 ++
 .../css-properties-values-api/resources/utils.js   | 237 +++++
 .../css/css-properties-values-api/self-utils.html  |  41 +
 .../css-properties-values-api/support/alt/alt.css  |   4 +
 .../css-properties-values-api/support/alt/alt.js   |  11 +
 .../support/main/main.css                          |  22 +
 .../css-properties-values-api/support/main/main.js |  25 +
 .../support/main/main.utf16be.css                  | Bin 0 -> 184 bytes
 .../css/css-properties-values-api/typedom.html     | 993 +++++++++++++++++++++
 .../css/css-properties-values-api/unit-cycles.html | 248 +++++
 .../css-properties-values-api/url-resolution.html  | 137 +++
 ...var-reference-registered-properties-cycles.html | 183 ++++
 .../var-reference-registered-properties.html       | 182 ++++
 90 files changed, 6797 insertions(+)
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/META.yml
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animate-invalid.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-custom-ident.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-image.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-inherited-used-by-standard-property.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-non-inherited-used-by-standard-property.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-comma-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-space-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-function.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-single-values.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-url.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-angle.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-color.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-custom-ident.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-image.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-inherited-used-by-standard-property.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-integer.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length-percentage.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-mismatched-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-non-inherited-used-by-standard-property.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-number.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-percentage.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-resolution.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-time.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-function.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-list.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-url.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-animation.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-cssom.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-non-matching-media-crash.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-shadow.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-stylesheets.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-typedom.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units-dynamic.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/at-property.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/conditional-rules.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/crashtests/initial-in-audio-crash.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/determine-registration.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/font-size-animation.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/idlharness.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/property-cascade.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/register-property-syntax-parsing.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/register-property.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/registered-properties-inheritance.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/registered-property-change-style-001.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/registered-property-computation.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/registered-property-crosstalk.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/registered-property-cssom.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/registered-property-initial.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/registered-property-revert.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/resources/utils.js
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/self-utils.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.css
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.js
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/support/main/main.css
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/support/main/main.js
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/support/main/main.utf16be.css
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/typedom.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/unit-cycles.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/url-resolution.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties-cycles.html
 create mode 100644 testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties.html

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

diff --git a/testing/web-platform/tests/css/css-properties-values-api/META.yml b/testing/web-platform/tests/css/css-properties-values-api/META.yml
new file mode 100644
index 0000000000..38cd166ca8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/META.yml
@@ -0,0 +1,4 @@
+spec: https://drafts.css-houdini.org/css-properties-values-api/
+suggested_reviewers:
+  - tabatkins
+  - astearns
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animate-invalid.html b/testing/web-platform/tests/css/css-properties-values-api/animate-invalid.html
new file mode 100644
index 0000000000..41cbd067b5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animate-invalid.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<title>Do not crash when animating to unresolved var()</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#dom-css-registerproperty">
+<link rel="help" href="https://crbug.com/1185524">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="target"></div>
+<script>
+  promise_test(async function(){
+    CSS.registerProperty({
+      name: '--x',
+      syntax: '<number>',
+      initialValue: '1',
+      inherits: false
+    });
+    let animation = target.animate({'--x': [ 'var(--unknown)']}, 100);
+    await animation.ready;
+    assert_equals(getComputedStyle(target).getPropertyValue('--x'), '1');
+  }, 'Do not crash when animating to unresolved var()');
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-comma-list.html
new file mode 100644
index 0000000000..a14b0bb09e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<angle>#",
+  inherits: false,
+  initialValue: "0deg"
+}, {
+  keyframes: ["100deg, 150deg", "200deg, 250deg"],
+  expected: "150deg, 200deg"
+}, 'Animating a custom property of type <angle>#');
+
+animation_test({
+  syntax: "<angle>#",
+  inherits: false,
+  initialValue: "100deg, 150deg"
+}, {
+  keyframes: "200deg, 250deg",
+  expected: "150deg, 200deg"
+}, 'Animating a custom property of type <angle># with a single keyframe');
+
+animation_test({
+  syntax: "<angle>#",
+  inherits: false,
+  initialValue: "50deg, 100deg"
+}, {
+  composite: "add",
+  keyframes: ["150deg, 200deg", "250deg, 300deg"],
+  expected: "250deg, 350deg"
+}, 'Animating a custom property of type <angle># with additivity');
+
+animation_test({
+  syntax: "<angle>#",
+  inherits: false,
+  initialValue: "50deg, 100deg"
+}, {
+  composite: "add",
+  keyframes: "150deg, 200deg",
+  expected: "125deg, 200deg"
+}, 'Animating a custom property of type <angle># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<angle>#",
+  inherits: false,
+  initialValue: "0deg, 0deg"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0deg, 50deg", "100deg, 100deg"],
+  expected: "250deg, 275deg"
+}, 'Animating a custom property of type <angle># with iterationComposite');
+
+discrete_animation_test("<angle>#", '10deg, 20deg', '30deg', 'Animating a custom property of type <angle># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-space-list.html
new file mode 100644
index 0000000000..8ce7a9deab
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<angle>+",
+  inherits: false,
+  initialValue: "0deg"
+}, {
+  keyframes: ["100deg 150deg", "200deg 250deg"],
+  expected: "150deg 200deg"
+}, 'Animating a custom property of type <angle>+');
+
+animation_test({
+  syntax: "<angle>+",
+  inherits: false,
+  initialValue: "100deg 150deg"
+}, {
+  keyframes: "200deg 250deg",
+  expected: "150deg 200deg"
+}, 'Animating a custom property of type <angle>+ with a single keyframe');
+
+animation_test({
+  syntax: "<angle>+",
+  inherits: false,
+  initialValue: "50deg 100deg"
+}, {
+  composite: "add",
+  keyframes: ["150deg 200deg", "250deg 300deg"],
+  expected: "250deg 350deg"
+}, 'Animating a custom property of type <angle>+ with additivity');
+
+animation_test({
+  syntax: "<angle>+",
+  inherits: false,
+  initialValue: "50deg 100deg"
+}, {
+  composite: "add",
+  keyframes: "150deg 200deg",
+  expected: "125deg 200deg"
+}, 'Animating a custom property of type <angle>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<angle>+",
+  inherits: false,
+  initialValue: "0deg 0deg"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0deg 50deg", "100deg 100deg"],
+  expected: "250deg 275deg"
+}, 'Animating a custom property of type <angle>+ with iterationComposite');
+
+discrete_animation_test("<angle>+", '10deg 20deg', '30deg', 'Animating a custom property of type <angle>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle.html
new file mode 100644
index 0000000000..0cb06aef0b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-angle.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<angle>",
+  inherits: false,
+  initialValue: "0deg"
+}, {
+  keyframes: ["100deg", "200deg"],
+  expected: "150deg"
+}, 'Animating a custom property of type <angle>');
+
+animation_test({
+  syntax: "<angle>",
+  inherits: false,
+  initialValue: "100deg"
+}, {
+  keyframes: "200deg",
+  expected: "150deg"
+}, 'Animating a custom property of type <angle> with a single keyframe');
+
+animation_test({
+  syntax: "<angle>",
+  inherits: false,
+  initialValue: "100deg"
+}, {
+  composite: "add",
+  keyframes: ["200deg", "300deg"],
+  expected: "350deg"
+}, 'Animating a custom property of type <angle> with additivity');
+
+animation_test({
+  syntax: "<angle>",
+  inherits: false,
+  initialValue: "100deg"
+}, {
+  composite: "add",
+  keyframes: "300deg",
+  expected: "250deg"
+}, 'Animating a custom property of type <angle> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<angle>",
+  inherits: false,
+  initialValue: "100deg"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0deg", "100deg"],
+  expected: "250deg"
+}, 'Animating a custom property of type <angle> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-comma-list.html
new file mode 100644
index 0000000000..7148df0724
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<color>#",
+  inherits: false,
+  initialValue: "black"
+}, {
+  keyframes: ["rgb(100, 100, 100), rgb(150, 150, 150)", "rgb(200, 200, 200), rgb(250, 250, 250)"],
+  expected: "rgb(150, 150, 150), rgb(200, 200, 200)"
+}, 'Animating a custom property of type <color>#');
+
+animation_test({
+  syntax: "<color>#",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100), rgb(150, 150, 150)"
+}, {
+  keyframes: "rgb(200, 200, 200), rgb(250, 250, 250)",
+  expected: "rgb(150, 150, 150), rgb(200, 200, 200)"
+}, 'Animating a custom property of type <color># with a single keyframe');
+
+animation_test({
+  syntax: "<color>#",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100), rgb(150, 150, 150)"
+}, {
+  composite: "add",
+  keyframes: ["rgb(25, 25, 25), rgb(50, 50, 50)", "rgb(75, 75, 75), rgb(100, 100, 100)"],
+  expected: "rgb(150, 150, 150), rgb(225, 225, 225)"
+}, 'Animating a custom property of type <color># with additivity');
+
+animation_test({
+  syntax: "<color>#",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100), rgb(150, 150, 150)"
+}, {
+  composite: "add",
+  keyframes: "rgb(50, 50, 50), rgb(100, 100, 100)",
+  expected: "rgb(125, 125, 125), rgb(200, 200, 200)"
+}, 'Animating a custom property of type <color># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<color>#",
+  inherits: false,
+  initialValue: "black"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["rgb(0, 0, 0), rgb(0, 0, 0)", "rgb(50, 50, 50), rgb(100, 100, 100)"],
+  expected: "rgb(125, 125, 125), rgb(250, 250, 250)"
+}, 'Animating a custom property of type <color># with iterationComposite');
+
+discrete_animation_test("<color>#", 'rgb(255, 0, 0), rgb(0, 255, 0)', 'rgb(0, 0, 255)', 'Animating a custom property of type <color># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-space-list.html
new file mode 100644
index 0000000000..679e244667
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<color>+",
+  inherits: false,
+  initialValue: "black"
+}, {
+  keyframes: ["rgb(100, 100, 100) rgb(150, 150, 150)", "rgb(200, 200, 200) rgb(250, 250, 250)"],
+  expected: "rgb(150, 150, 150) rgb(200, 200, 200)"
+}, 'Animating a custom property of type <color>+');
+
+animation_test({
+  syntax: "<color>+",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100) rgb(150, 150, 150)"
+}, {
+  keyframes: "rgb(200, 200, 200) rgb(250, 250, 250)",
+  expected: "rgb(150, 150, 150) rgb(200, 200, 200)"
+}, 'Animating a custom property of type <color>+ with a single keyframe');
+
+animation_test({
+  syntax: "<color>+",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100) rgb(150, 150, 150)"
+}, {
+  composite: "add",
+  keyframes: ["rgb(25, 25, 25) rgb(50, 50, 50)", "rgb(75, 75, 75) rgb(100, 100, 100)"],
+  expected: "rgb(150, 150, 150) rgb(225, 225, 225)"
+}, 'Animating a custom property of type <color>+ with additivity');
+
+animation_test({
+  syntax: "<color>+",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100) rgb(150, 150, 150)"
+}, {
+  composite: "add",
+  keyframes: "rgb(50, 50, 50) rgb(100, 100, 100)",
+  expected: "rgb(125, 125, 125) rgb(200, 200, 200)"
+}, 'Animating a custom property of type <color>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<color>+",
+  inherits: false,
+  initialValue: "black"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["rgb(0, 0, 0) rgb(0, 0, 0)", "rgb(50, 50, 50) rgb(100, 100, 100)"],
+  expected: "rgb(125, 125, 125) rgb(250, 250, 250)"
+}, 'Animating a custom property of type <color>+ with iterationComposite');
+
+discrete_animation_test("<color>+", 'rgb(255, 0, 0) rgb(0, 255, 0)', 'rgb(0, 0, 255)', 'Animating a custom property of type <color>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color.html
new file mode 100644
index 0000000000..001fc407e7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-color.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<color>",
+  inherits: false,
+  initialValue: "black"
+}, {
+  keyframes: ["rgb(100, 100, 100)", "rgb(200, 200, 200)"],
+  expected: "rgb(150, 150, 150)"
+}, 'Animating a custom property of type <color>');
+
+animation_test({
+  syntax: "<color>",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100)"
+}, {
+  keyframes: "rgb(200, 200, 200)",
+  expected: "rgb(150, 150, 150)"
+}, 'Animating a custom property of type <color> with a single keyframe');
+
+animation_test({
+  syntax: "<color>",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100)"
+}, {
+  composite: "add",
+  keyframes: ["rgb(50, 50, 50)", "rgb(150, 150, 150)"],
+  expected: "rgb(200, 200, 200)"
+}, 'Animating a custom property of type <color> with additivity');
+
+animation_test({
+  syntax: "<color>",
+  inherits: false,
+  initialValue: "rgb(100, 100, 100)"
+}, {
+  composite: "add",
+  keyframes: "rgb(150, 150, 150)",
+  expected: "rgb(175, 175, 175)"
+}, 'Animating a custom property of type <color> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<color>",
+  inherits: false,
+  initialValue: "black"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["rgb(0, 0, 0)", "rgb(100, 100, 100)"],
+  expected: "rgb(250, 250, 250)"
+}, 'Animating a custom property of type <color> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-custom-ident.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-custom-ident.html
new file mode 100644
index 0000000000..5bc2bed343
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-custom-ident.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+discrete_animation_test("<custom-ident>", "from", "to");
+discrete_animation_test("<custom-ident>+", "from1 from2", "to1 to2");
+discrete_animation_test("<custom-ident>#", "from1, from2", "to1, to2");
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-image.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-image.html
new file mode 100644
index 0000000000..4f9505f9a8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-image.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+discrete_animation_test("<image>", 'url("https://example.com/from")', 'url("https://example.com/to")');
+discrete_animation_test("<image>+", 'url("https://example.com/from1") url("https://example.com/from2")', 'url("https://example.com/to1") url("https://example.com/to2")');
+discrete_animation_test("<image>#", 'url("https://example.com/from1"), url("https://example.com/from2")', 'url("https://example.com/to1"), url("https://example.com/to2")');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-inherited-used-by-standard-property.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-inherited-used-by-standard-property.html
new file mode 100644
index 0000000000..231ecca8c7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-inherited-used-by-standard-property.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="container">
+    <div id="target"></div>
+</div>
+<script>
+
+test(() => {
+  CSS.registerProperty({
+    name: "--my-length",
+    syntax: "<length>",
+    inherits: true,
+    initialValue: "0px"
+  });
+
+  target.style.marginLeft = "var(--my-length)";
+
+  const duration = 1000;
+  const animation = container.animate({ "--my-length": "100px" }, duration);
+  animation.pause();
+  animation.currentTime = duration / 4;
+
+  assert_equals(getComputedStyle(target).marginLeft, "25px");
+}, "Animating an inherited CSS variable on a parent is reflected on a standard property using that variable as a value on a child");
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-comma-list.html
new file mode 100644
index 0000000000..3f4beaedf1
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<integer>#",
+  inherits: false,
+  initialValue: "0"
+}, {
+  keyframes: ["100, 150", "200, 250"],
+  expected: "150, 200"
+}, 'Animating a custom property of type <integer>#');
+
+animation_test({
+  syntax: "<integer>#",
+  inherits: false,
+  initialValue: "100, 150"
+}, {
+  keyframes: "200, 250",
+  expected: "150, 200"
+}, 'Animating a custom property of type <integer># with a single keyframe');
+
+animation_test({
+  syntax: "<integer>#",
+  inherits: false,
+  initialValue: "50, 100"
+}, {
+  composite: "add",
+  keyframes: ["150, 200", "250, 300"],
+  expected: "250, 350"
+}, 'Animating a custom property of type <integer># with additivity');
+
+animation_test({
+  syntax: "<integer>#",
+  inherits: false,
+  initialValue: "50, 100"
+}, {
+  composite: "add",
+  keyframes: "150, 200",
+  expected: "125, 200"
+}, 'Animating a custom property of type <integer># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<integer>#",
+  inherits: false,
+  initialValue: "0, 0"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0, 50", "100, 100"],
+  expected: "250, 275"
+}, 'Animating a custom property of type <integer># with iterationComposite');
+
+discrete_animation_test("<integer>#", '10, 20', '30', 'Animating a custom property of type <integer># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-space-list.html
new file mode 100644
index 0000000000..89feb3af15
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<integer>+",
+  inherits: false,
+  initialValue: "0"
+}, {
+  keyframes: ["100 150", "200 250"],
+  expected: "150 200"
+}, 'Animating a custom property of type <integer>+');
+
+animation_test({
+  syntax: "<integer>+",
+  inherits: false,
+  initialValue: "100 150"
+}, {
+  keyframes: "200 250",
+  expected: "150 200"
+}, 'Animating a custom property of type <integer>+ with a single keyframe');
+
+animation_test({
+  syntax: "<integer>+",
+  inherits: false,
+  initialValue: "50 100"
+}, {
+  composite: "add",
+  keyframes: ["150 200", "250 300"],
+  expected: "250 350"
+}, 'Animating a custom property of type <integer>+ with additivity');
+
+animation_test({
+  syntax: "<integer>+",
+  inherits: false,
+  initialValue: "50 100"
+}, {
+  composite: "add",
+  keyframes: "150 200",
+  expected: "125 200"
+}, 'Animating a custom property of type <integer>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<integer>+",
+  inherits: false,
+  initialValue: "0 0"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0 50", "100 100"],
+  expected: "250 275"
+}, 'Animating a custom property of type <integer>+ with iterationComposite');
+
+discrete_animation_test("<integer>+", '10 20', '30', 'Animating a custom property of type <integer>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer.html
new file mode 100644
index 0000000000..400308a92e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-integer.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<integer>",
+  inherits: false,
+  initialValue: 0
+}, {
+  keyframes: [100, 200],
+  expected: "150"
+}, 'Animating a custom property of type <integer>');
+
+animation_test({
+  syntax: "<integer>",
+  inherits: false,
+  initialValue: 100
+}, {
+  keyframes: 200,
+  expected: "150"
+}, 'Animating a custom property of type <integer> with a single keyframe');
+
+animation_test({
+  syntax: "<integer>",
+  inherits: false,
+  initialValue: 100
+}, {
+  composite: "add",
+  keyframes: [200, 300],
+  expected: "350"
+}, 'Animating a custom property of type <integer> with additivity');
+
+animation_test({
+  syntax: "<integer>",
+  inherits: false,
+  initialValue: 100
+}, {
+  composite: "add",
+  keyframes: 300,
+  expected: "250"
+}, 'Animating a custom property of type <integer> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<integer>",
+  inherits: false,
+  initialValue: 100
+}, {
+  iterationComposite: "accumulate",
+  keyframes: [0, 100],
+  expected: "250"
+}, 'Animating a custom property of type <integer> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-comma-list.html
new file mode 100644
index 0000000000..6f1fefb76f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<length>#",
+  inherits: false,
+  initialValue: "0px"
+}, {
+  keyframes: ["100px, 150px", "200px, 250px"],
+  expected: "150px, 200px"
+}, 'Animating a custom property of type <length>#');
+
+animation_test({
+  syntax: "<length>#",
+  inherits: false,
+  initialValue: "100px, 150px"
+}, {
+  keyframes: "200px, 250px",
+  expected: "150px, 200px"
+}, 'Animating a custom property of type <length># with a single keyframe');
+
+animation_test({
+  syntax: "<length>#",
+  inherits: false,
+  initialValue: "50px, 100px"
+}, {
+  composite: "add",
+  keyframes: ["150px, 200px", "250px, 300px"],
+  expected: "250px, 350px"
+}, 'Animating a custom property of type <length># with additivity');
+
+animation_test({
+  syntax: "<length>#",
+  inherits: false,
+  initialValue: "50px, 100px"
+}, {
+  composite: "add",
+  keyframes: "150px, 200px",
+  expected: "125px, 200px"
+}, 'Animating a custom property of type <length># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<length>#",
+  inherits: false,
+  initialValue: "0px, 0px"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0px, 50px", "100px, 100px"],
+  expected: "250px, 275px"
+}, 'Animating a custom property of type <length># with iterationComposite');
+
+discrete_animation_test("<length>#", '10px, 20px', '30px', 'Animating a custom property of type <length># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-comma-list.html
new file mode 100644
index 0000000000..bf3e75b791
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<length-percentage>#",
+  inherits: false,
+  initialValue: "0px"
+}, {
+  keyframes: ["100px, 150%", "200%, 250px"],
+  expected: "calc(100% + 50px), calc(75% + 125px)"
+}, 'Animating a custom property of type <length-percentage>#');
+
+animation_test({
+  syntax: "<length-percentage>#",
+  inherits: false,
+  initialValue: "100px, 150%"
+}, {
+  keyframes: "200%, 250px",
+  expected: "calc(100% + 50px), calc(75% + 125px)"
+}, 'Animating a custom property of type <length-percentage># with a single keyframe');
+
+animation_test({
+  syntax: "<length-percentage>#",
+  inherits: false,
+  initialValue: "50px, 100%"
+}, {
+  composite: "add",
+  keyframes: ["150%, 200px", "250%, 300px"],
+  expected: "calc(200% + 50px), calc(100% + 250px)"
+}, 'Animating a custom property of type <length-percentage># with additivity');
+
+animation_test({
+  syntax: "<length-percentage>#",
+  inherits: false,
+  initialValue: "50px, 100%"
+}, {
+  composite: "add",
+  keyframes: "150%, 200px",
+  expected: "calc(75% + 50px), calc(100% + 100px)"
+}, 'Animating a custom property of type <length-percentage># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<length-percentage>#",
+  inherits: false,
+  initialValue: "0px, 0px"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0px, 50%", "100%, 100px"],
+  expected: "250%, calc(25% + 50px)"
+}, 'Animating a custom property of type <length-percentage># with iterationComposite');
+
+discrete_animation_test("<length-percentage>#", '10px, 20%', '30px', 'Animating a custom property of type <length-percentage># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-space-list.html
new file mode 100644
index 0000000000..9497093b6e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<length-percentage>+",
+  inherits: false,
+  initialValue: "0px"
+}, {
+  keyframes: ["100px 150%", "200% 250px"],
+  expected: "calc(100% + 50px) calc(75% + 125px)"
+}, 'Animating a custom property of type <length-percentage>+');
+
+animation_test({
+  syntax: "<length-percentage>+",
+  inherits: false,
+  initialValue: "100px 150%"
+}, {
+  keyframes: "200% 250px",
+  expected: "calc(100% + 50px) calc(75% + 125px)"
+}, 'Animating a custom property of type <length-percentage>+ with a single keyframe');
+
+animation_test({
+  syntax: "<length-percentage>+",
+  inherits: false,
+  initialValue: "50px 100%"
+}, {
+  composite: "add",
+  keyframes: ["150% 200px", "250% 300px"],
+  expected: "calc(200% + 50px) calc(100% + 250px)"
+}, 'Animating a custom property of type <length-percentage>+ with additivity');
+
+animation_test({
+  syntax: "<length-percentage>+",
+  inherits: false,
+  initialValue: "50px 100%"
+}, {
+  composite: "add",
+  keyframes: "150% 200px",
+  expected: "calc(75% + 50px) calc(100% + 100px)"
+}, 'Animating a custom property of type <length-percentage>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<length-percentage>+",
+  inherits: false,
+  initialValue: "0px 0px"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0px 50%", "100% 100px"],
+  expected: "250% calc(25% + 50px)"
+}, 'Animating a custom property of type <length-percentage>+ with iterationComposite');
+
+discrete_animation_test("<length-percentage>+", '10px 20%', '30px', 'Animating a custom property of type <length-percentage>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage.html
new file mode 100644
index 0000000000..096efd6ba7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-percentage.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<length-percentage>",
+  inherits: false,
+  initialValue: "0px"
+}, {
+  keyframes: ["100px", "100%"],
+  expected: "calc(50% + 50px)"
+}, 'Animating a custom property of type <length-percentage>');
+
+animation_test({
+  syntax: "<length-percentage>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  keyframes: "100%",
+  expected: "calc(50% + 50px)"
+}, 'Animating a custom property of type <length-percentage> with a single keyframe');
+
+animation_test({
+  syntax: "<length-percentage>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  composite: "add",
+  keyframes: ["200%", "300%"],
+  expected: "calc(250% + 100px)"
+}, 'Animating a custom property of type <length-percentage> with additivity');
+
+animation_test({
+  syntax: "<length-percentage>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  composite: "add",
+  keyframes: "300%",
+  expected: "calc(150% + 100px)"
+}, 'Animating a custom property of type <length-percentage> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<length-percentage>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["50px", "100%"],
+  expected: "calc(250% + 25px)"
+}, 'Animating a custom property of type <length-percentage> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-space-list.html
new file mode 100644
index 0000000000..102259a070
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<length>+",
+  inherits: false,
+  initialValue: "0px"
+}, {
+  keyframes: ["100px 150px", "200px 250px"],
+  expected: "150px 200px"
+}, 'Animating a custom property of type <length>+');
+
+animation_test({
+  syntax: "<length>+",
+  inherits: false,
+  initialValue: "100px 150px"
+}, {
+  keyframes: "200px 250px",
+  expected: "150px 200px"
+}, 'Animating a custom property of type <length>+ with a single keyframe');
+
+animation_test({
+  syntax: "<length>+",
+  inherits: false,
+  initialValue: "50px 100px"
+}, {
+  composite: "add",
+  keyframes: ["150px 200px", "250px 300px"],
+  expected: "250px 350px"
+}, 'Animating a custom property of type <length>+ with additivity');
+
+animation_test({
+  syntax: "<length>+",
+  inherits: false,
+  initialValue: "50px 100px"
+}, {
+  composite: "add",
+  keyframes: "150px 200px",
+  expected: "125px 200px"
+}, 'Animating a custom property of type <length>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<length>+",
+  inherits: false,
+  initialValue: "0px 0px"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0px 50px", "100px 100px"],
+  expected: "250px 275px"
+}, 'Animating a custom property of type <length>+ with iterationComposite');
+
+discrete_animation_test("<length>+", '10px 20px', '30px', 'Animating a custom property of type <length>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length.html
new file mode 100644
index 0000000000..8849bf52d6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-length.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<length>",
+  inherits: false,
+  initialValue: "0px"
+}, {
+  keyframes: ["100px", "200px"],
+  expected: "150px"
+}, 'Animating a custom property of type <length>');
+
+animation_test({
+  syntax: "<length>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  keyframes: "200px",
+  expected: "150px"
+}, 'Animating a custom property of type <length> with a single keyframe');
+
+animation_test({
+  syntax: "<length>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  composite: "add",
+  keyframes: ["200px", "300px"],
+  expected: "350px"
+}, 'Animating a custom property of type <length> with additivity');
+
+animation_test({
+  syntax: "<length>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  composite: "add",
+  keyframes: "300px",
+  expected: "250px"
+}, 'Animating a custom property of type <length> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<length>",
+  inherits: false,
+  initialValue: "100px"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0px", "100px"],
+  expected: "250px"
+}, 'Animating a custom property of type <length> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-non-inherited-used-by-standard-property.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-non-inherited-used-by-standard-property.html
new file mode 100644
index 0000000000..11d3f6740d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-non-inherited-used-by-standard-property.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+test(() => {
+  CSS.registerProperty({
+    name: "--my-length",
+    syntax: "<length>",
+    inherits: false,
+    initialValue: "0px"
+  });
+
+  target.style.marginLeft = "var(--my-length)";
+
+  const duration = 1000;
+  const animation = target.animate({ "--my-length": "100px" }, duration);
+  animation.pause();
+  animation.currentTime = duration / 4;
+
+  assert_equals(getComputedStyle(target).marginLeft, "25px");
+}, "Animating a non-inherited CSS variable is reflected on a standard property using that variable as a value");
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-comma-list.html
new file mode 100644
index 0000000000..937b6caeeb
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<number>#",
+  inherits: false,
+  initialValue: "0"
+}, {
+  keyframes: ["100, 150", "200, 250"],
+  expected: "150, 200"
+}, 'Animating a custom property of type <number>#');
+
+animation_test({
+  syntax: "<number>#",
+  inherits: false,
+  initialValue: "100, 150"
+}, {
+  keyframes: "200, 250",
+  expected: "150, 200"
+}, 'Animating a custom property of type <number># with a single keyframe');
+
+animation_test({
+  syntax: "<number>#",
+  inherits: false,
+  initialValue: "50, 100"
+}, {
+  composite: "add",
+  keyframes: ["150, 200", "250, 300"],
+  expected: "250, 350"
+}, 'Animating a custom property of type <number># with additivity');
+
+animation_test({
+  syntax: "<number>#",
+  inherits: false,
+  initialValue: "50, 100"
+}, {
+  composite: "add",
+  keyframes: "150, 200",
+  expected: "125, 200"
+}, 'Animating a custom property of type <number># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<number>#",
+  inherits: false,
+  initialValue: "0, 0"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0, 50", "100, 100"],
+  expected: "250, 275"
+}, 'Animating a custom property of type <number># with iterationComposite');
+
+discrete_animation_test("<number>#", '10, 20', '30', 'Animating a custom property of type <number># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-space-list.html
new file mode 100644
index 0000000000..61f177eb05
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<number>+",
+  inherits: false,
+  initialValue: "0"
+}, {
+  keyframes: ["100 150", "200 250"],
+  expected: "150 200"
+}, 'Animating a custom property of type <number>+');
+
+animation_test({
+  syntax: "<number>+",
+  inherits: false,
+  initialValue: "100 150"
+}, {
+  keyframes: "200 250",
+  expected: "150 200"
+}, 'Animating a custom property of type <number>+ with a single keyframe');
+
+animation_test({
+  syntax: "<number>+",
+  inherits: false,
+  initialValue: "50 100"
+}, {
+  composite: "add",
+  keyframes: ["150 200", "250 300"],
+  expected: "250 350"
+}, 'Animating a custom property of type <number>+ with additivity');
+
+animation_test({
+  syntax: "<number>+",
+  inherits: false,
+  initialValue: "50 100"
+}, {
+  composite: "add",
+  keyframes: "150 200",
+  expected: "125 200"
+}, 'Animating a custom property of type <number>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<number>+",
+  inherits: false,
+  initialValue: "0 0"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0 50", "100 100"],
+  expected: "250 275"
+}, 'Animating a custom property of type <number>+ with iterationComposite');
+
+discrete_animation_test("<number>+", '10 20', '30', 'Animating a custom property of type <number>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number.html
new file mode 100644
index 0000000000..ea28acacf6
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-number.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<number>",
+  inherits: false,
+  initialValue: 0
+}, {
+  keyframes: [100, 200],
+  expected: "150"
+}, 'Animating a custom property of type <number>');
+
+animation_test({
+  syntax: "<number>",
+  inherits: false,
+  initialValue: 100
+}, {
+  keyframes: 200,
+  expected: "150"
+}, 'Animating a custom property of type <number> with a single keyframe');
+
+animation_test({
+  syntax: "<number>",
+  inherits: false,
+  initialValue: 100
+}, {
+  composite: "add",
+  keyframes: [200, 300],
+  expected: "350"
+}, 'Animating a custom property of type <number> with additivity');
+
+animation_test({
+  syntax: "<number>",
+  inherits: false,
+  initialValue: 100
+}, {
+  composite: "add",
+  keyframes: 300,
+  expected: "250"
+}, 'Animating a custom property of type <number> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<number>",
+  inherits: false,
+  initialValue: 100
+}, {
+  iterationComposite: "accumulate",
+  keyframes: [0, 100],
+  expected: "250"
+}, 'Animating a custom property of type <number> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-comma-list.html
new file mode 100644
index 0000000000..c690046646
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<percentage>#",
+  inherits: false,
+  initialValue: "0%"
+}, {
+  keyframes: ["100%, 150%", "200%, 250%"],
+  expected: "150%, 200%"
+}, 'Animating a custom property of type <percentage>#');
+
+animation_test({
+  syntax: "<percentage>#",
+  inherits: false,
+  initialValue: "100%, 150%"
+}, {
+  keyframes: "200%, 250%",
+  expected: "150%, 200%"
+}, 'Animating a custom property of type <percentage># with a single keyframe');
+
+animation_test({
+  syntax: "<percentage>#",
+  inherits: false,
+  initialValue: "50%, 100%"
+}, {
+  composite: "add",
+  keyframes: ["150%, 200%", "250%, 300%"],
+  expected: "250%, 350%"
+}, 'Animating a custom property of type <percentage># with additivity');
+
+animation_test({
+  syntax: "<percentage>#",
+  inherits: false,
+  initialValue: "50%, 100%"
+}, {
+  composite: "add",
+  keyframes: "150%, 200%",
+  expected: "125%, 200%"
+}, 'Animating a custom property of type <percentage># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<percentage>#",
+  inherits: false,
+  initialValue: "0%, 0%"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0%, 50%", "100%, 100%"],
+  expected: "250%, 275%"
+}, 'Animating a custom property of type <percentage># with iterationComposite');
+
+discrete_animation_test("<percentage>#", '10%, 20%', '30%', 'Animating a custom property of type <percentage># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-space-list.html
new file mode 100644
index 0000000000..d012b851fd
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<percentage>+",
+  inherits: false,
+  initialValue: "0%"
+}, {
+  keyframes: ["100% 150%", "200% 250%"],
+  expected: "150% 200%"
+}, 'Animating a custom property of type <percentage>+');
+
+animation_test({
+  syntax: "<percentage>+",
+  inherits: false,
+  initialValue: "100% 150%"
+}, {
+  keyframes: "200% 250%",
+  expected: "150% 200%"
+}, 'Animating a custom property of type <percentage>+ with a single keyframe');
+
+animation_test({
+  syntax: "<percentage>+",
+  inherits: false,
+  initialValue: "50% 100%"
+}, {
+  composite: "add",
+  keyframes: ["150% 200%", "250% 300%"],
+  expected: "250% 350%"
+}, 'Animating a custom property of type <percentage>+ with additivity');
+
+animation_test({
+  syntax: "<percentage>+",
+  inherits: false,
+  initialValue: "50% 100%"
+}, {
+  composite: "add",
+  keyframes: "150% 200%",
+  expected: "125% 200%"
+}, 'Animating a custom property of type <percentage>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<percentage>+",
+  inherits: false,
+  initialValue: "0% 0%"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0% 50%", "100% 100%"],
+  expected: "250% 275%"
+}, 'Animating a custom property of type <percentage>+ with iterationComposite');
+
+discrete_animation_test("<percentage>+", '10% 20%', '30%', 'Animating a custom property of type <percentage>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage.html
new file mode 100644
index 0000000000..6af498e702
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-percentage.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<percentage>",
+  inherits: false,
+  initialValue: "0%"
+}, {
+  keyframes: ["100%", "200%"],
+  expected: "150%"
+}, 'Animating a custom property of type <percentage>');
+
+animation_test({
+  syntax: "<percentage>",
+  inherits: false,
+  initialValue: "100%"
+}, {
+  keyframes: "200%",
+  expected: "150%"
+}, 'Animating a custom property of type <percentage> with a single keyframe');
+
+animation_test({
+  syntax: "<percentage>",
+  inherits: false,
+  initialValue: "100%"
+}, {
+  composite: "add",
+  keyframes: ["200%", "300%"],
+  expected: "350%"
+}, 'Animating a custom property of type <percentage> with additivity');
+
+animation_test({
+  syntax: "<percentage>",
+  inherits: false,
+  initialValue: "100%"
+}, {
+  composite: "add",
+  keyframes: "300%",
+  expected: "250%"
+}, 'Animating a custom property of type <percentage> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<percentage>",
+  inherits: false,
+  initialValue: "100%"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0%", "100%"],
+  expected: "250%"
+}, 'Animating a custom property of type <percentage> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-comma-list.html
new file mode 100644
index 0000000000..22201947ff
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<resolution>#",
+  inherits: false,
+  initialValue: "0dppx"
+}, {
+  keyframes: ["100dppx, 150dppx", "200dppx, 250dppx"],
+  expected: "150dppx, 200dppx"
+}, 'Animating a custom property of type <resolution>#');
+
+animation_test({
+  syntax: "<resolution>#",
+  inherits: false,
+  initialValue: "100dppx, 150dppx"
+}, {
+  keyframes: "200dppx, 250dppx",
+  expected: "150dppx, 200dppx"
+}, 'Animating a custom property of type <resolution># with a single keyframe');
+
+animation_test({
+  syntax: "<resolution>#",
+  inherits: false,
+  initialValue: "50dppx, 100dppx"
+}, {
+  composite: "add",
+  keyframes: ["150dppx, 200dppx", "250dppx, 300dppx"],
+  expected: "250dppx, 350dppx"
+}, 'Animating a custom property of type <resolution># with additivity');
+
+animation_test({
+  syntax: "<resolution>#",
+  inherits: false,
+  initialValue: "50dppx, 100dppx"
+}, {
+  composite: "add",
+  keyframes: "150dppx, 200dppx",
+  expected: "125dppx, 200dppx"
+}, 'Animating a custom property of type <resolution># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<resolution>#",
+  inherits: false,
+  initialValue: "0dppx, 0dppx"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0dppx, 50dppx", "100dppx, 100dppx"],
+  expected: "250dppx, 275dppx"
+}, 'Animating a custom property of type <resolution># with iterationComposite');
+
+discrete_animation_test("<resolution>#", '10dppx, 20dppx', '30dppx', 'Animating a custom property of type <resolution># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-space-list.html
new file mode 100644
index 0000000000..d4765d4e32
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<resolution>+",
+  inherits: false,
+  initialValue: "0dppx"
+}, {
+  keyframes: ["100dppx 150dppx", "200dppx 250dppx"],
+  expected: "150dppx 200dppx"
+}, 'Animating a custom property of type <resolution>+');
+
+animation_test({
+  syntax: "<resolution>+",
+  inherits: false,
+  initialValue: "100dppx 150dppx"
+}, {
+  keyframes: "200dppx 250dppx",
+  expected: "150dppx 200dppx"
+}, 'Animating a custom property of type <resolution>+ with a single keyframe');
+
+animation_test({
+  syntax: "<resolution>+",
+  inherits: false,
+  initialValue: "50dppx 100dppx"
+}, {
+  composite: "add",
+  keyframes: ["150dppx 200dppx", "250dppx 300dppx"],
+  expected: "250dppx 350dppx"
+}, 'Animating a custom property of type <resolution>+ with additivity');
+
+animation_test({
+  syntax: "<resolution>+",
+  inherits: false,
+  initialValue: "50dppx 100dppx"
+}, {
+  composite: "add",
+  keyframes: "150dppx 200dppx",
+  expected: "125dppx 200dppx"
+}, 'Animating a custom property of type <resolution>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<resolution>+",
+  inherits: false,
+  initialValue: "0dppx 0dppx"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0dppx 50dppx", "100dppx 100dppx"],
+  expected: "250dppx 275dppx"
+}, 'Animating a custom property of type <resolution>+ with iterationComposite');
+
+discrete_animation_test("<resolution>+", '10dppx 20dppx', '30dppx', 'Animating a custom property of type <resolution>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution.html
new file mode 100644
index 0000000000..3d05139edc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-resolution.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<resolution>",
+  inherits: false,
+  initialValue: "0dppx"
+}, {
+  keyframes: ["100dppx", "200dppx"],
+  expected: "150dppx"
+}, 'Animating a custom property of type <resolution>');
+
+animation_test({
+  syntax: "<resolution>",
+  inherits: false,
+  initialValue: "100dppx"
+}, {
+  keyframes: "200dppx",
+  expected: "150dppx"
+}, 'Animating a custom property of type <resolution> with a single keyframe');
+
+animation_test({
+  syntax: "<resolution>",
+  inherits: false,
+  initialValue: "100dppx"
+}, {
+  composite: "add",
+  keyframes: ["200dppx", "300dppx"],
+  expected: "350dppx"
+}, 'Animating a custom property of type <resolution> with additivity');
+
+animation_test({
+  syntax: "<resolution>",
+  inherits: false,
+  initialValue: "100dppx"
+}, {
+  composite: "add",
+  keyframes: "300dppx",
+  expected: "250dppx"
+}, 'Animating a custom property of type <resolution> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<resolution>",
+  inherits: false,
+  initialValue: "100dppx"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0dppx", "100dppx"],
+  expected: "250dppx"
+}, 'Animating a custom property of type <resolution> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-comma-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-comma-list.html
new file mode 100644
index 0000000000..bf07baf130
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-comma-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<time>#",
+  inherits: false,
+  initialValue: "0s"
+}, {
+  keyframes: ["100s, 150s", "200s, 250s"],
+  expected: "150s, 200s"
+}, 'Animating a custom property of type <time>#');
+
+animation_test({
+  syntax: "<time>#",
+  inherits: false,
+  initialValue: "100s, 150s"
+}, {
+  keyframes: "200s, 250s",
+  expected: "150s, 200s"
+}, 'Animating a custom property of type <time># with a single keyframe');
+
+animation_test({
+  syntax: "<time>#",
+  inherits: false,
+  initialValue: "50s, 100s"
+}, {
+  composite: "add",
+  keyframes: ["150s, 200s", "250s, 300s"],
+  expected: "250s, 350s"
+}, 'Animating a custom property of type <time># with additivity');
+
+animation_test({
+  syntax: "<time>#",
+  inherits: false,
+  initialValue: "50s, 100s"
+}, {
+  composite: "add",
+  keyframes: "150s, 200s",
+  expected: "125s, 200s"
+}, 'Animating a custom property of type <time># with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<time>#",
+  inherits: false,
+  initialValue: "0s, 0s"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0s, 50s", "100s, 100s"],
+  expected: "250s, 275s"
+}, 'Animating a custom property of type <time># with iterationComposite');
+
+discrete_animation_test("<time>#", '10s, 20s', '30s', 'Animating a custom property of type <time># with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-space-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-space-list.html
new file mode 100644
index 0000000000..c0fe206ea7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time-space-list.html
@@ -0,0 +1,59 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<time>+",
+  inherits: false,
+  initialValue: "0s"
+}, {
+  keyframes: ["100s 150s", "200s 250s"],
+  expected: "150s 200s"
+}, 'Animating a custom property of type <time>+');
+
+animation_test({
+  syntax: "<time>+",
+  inherits: false,
+  initialValue: "100s 150s"
+}, {
+  keyframes: "200s 250s",
+  expected: "150s 200s"
+}, 'Animating a custom property of type <time>+ with a single keyframe');
+
+animation_test({
+  syntax: "<time>+",
+  inherits: false,
+  initialValue: "50s 100s"
+}, {
+  composite: "add",
+  keyframes: ["150s 200s", "250s 300s"],
+  expected: "250s 350s"
+}, 'Animating a custom property of type <time>+ with additivity');
+
+animation_test({
+  syntax: "<time>+",
+  inherits: false,
+  initialValue: "50s 100s"
+}, {
+  composite: "add",
+  keyframes: "150s 200s",
+  expected: "125s 200s"
+}, 'Animating a custom property of type <time>+ with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<time>+",
+  inherits: false,
+  initialValue: "0s 0s"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0s 50s", "100s 100s"],
+  expected: "250s 275s"
+}, 'Animating a custom property of type <time>+ with iterationComposite');
+
+discrete_animation_test("<time>+", '10s 20s', '30s', 'Animating a custom property of type <time>+ with different lengths is discrete');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time.html
new file mode 100644
index 0000000000..b9f5984db7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-time.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<time>",
+  inherits: false,
+  initialValue: "0s"
+}, {
+  keyframes: ["100s", "200s"],
+  expected: "150s"
+}, 'Animating a custom property of type <time>');
+
+animation_test({
+  syntax: "<time>",
+  inherits: false,
+  initialValue: "100s"
+}, {
+  keyframes: "200s",
+  expected: "150s"
+}, 'Animating a custom property of type <time> with a single keyframe');
+
+animation_test({
+  syntax: "<time>",
+  inherits: false,
+  initialValue: "100s"
+}, {
+  composite: "add",
+  keyframes: ["200s", "300s"],
+  expected: "350s"
+}, 'Animating a custom property of type <time> with additivity');
+
+animation_test({
+  syntax: "<time>",
+  inherits: false,
+  initialValue: "100s"
+}, {
+  composite: "add",
+  keyframes: "300s",
+  expected: "250s"
+}, 'Animating a custom property of type <time> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<time>",
+  inherits: false,
+  initialValue: "100s"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["0s", "100s"],
+  expected: "250s"
+}, 'Animating a custom property of type <time> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-function.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-function.html
new file mode 100644
index 0000000000..c4fcd5ce4c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-function.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<transform-function>",
+  inherits: false,
+  initialValue: "translateX(0px)"
+}, {
+  keyframes: ["translateX(100px)", "translateX(200px)"],
+  expected: "translateX(150px)"
+}, 'Animating a custom property of type <transform-function>');
+
+animation_test({
+  syntax: "<transform-function>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  keyframes: "translateX(200px)",
+  expected: "translateX(150px)"
+}, 'Animating a custom property of type <transform-function> with a single keyframe');
+
+animation_test({
+  syntax: "<transform-function>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  composite: "add",
+  keyframes: ["translateX(200px)", "translateX(300px)"],
+  expected: "translateX(350px)"
+}, 'Animating a custom property of type <transform-function> with additivity');
+
+animation_test({
+  syntax: "<transform-function>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  composite: "add",
+  keyframes: "translateX(300px)",
+  expected: "translateX(250px)"
+}, 'Animating a custom property of type <transform-function> with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<transform-function>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["translateX(0px)", "translateX(100px)"],
+  expected: "translateX(250px)"
+}, 'Animating a custom property of type <transform-function> with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
new file mode 100644
index 0000000000..9ffaec830f
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
@@ -0,0 +1,66 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(0px)"
+}, {
+  keyframes: ["translateX(100px) scale(2)", "translateX(200px) scale(4)"],
+  expected: "translateX(150px) scale(3)"
+}, 'Animating a custom property of type <transform-list> containing multiple values');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px) scale(2)"
+}, {
+  keyframes: "translateX(200px) scale(4)",
+  expected: "translateX(150px) scale(3)"
+}, 'Animating a custom property of type <transform-list> containing multiple values with a single keyframe');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px) scale(2)"
+}, {
+  composite: "add",
+  keyframes: ["translateX(200px) scale(3)", "translateX(300px) scale(4)"],
+  expected: "translateX(100px) scale(2) translateX(250px) scale(3.5)"
+}, 'Animating a custom property of type <transform-list> containing multiple values with additivity');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px) scale(2)"
+}, {
+  composite: "add",
+  keyframes: "translateX(300px) scale(3)",
+  expected: "translateX(100px) scale(2) translateX(150px) scale(2)"
+}, 'Animating a custom property of type <transform-list> containing multiple values with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px) scale(2)"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["translateX(0px) scale(3)", "translateX(100px) scale(4)"],
+  expected: "translateX(250px) scale(11.5)"
+}, 'Animating a custom property of type <transform-list> containing multiple values with iterationComposite');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(0px)"
+}, {
+  keyframes: ["translateX(100px) scale(2)", "rotate(180deg)"],
+  expected: "matrix(0, -1.5, 1.5, 0, 50, 0)"
+}, 'Animating a custom property of type <transform-list> containing multiple values and with mismatching list lengths');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-single-values.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-single-values.html
new file mode 100644
index 0000000000..b9bce6dd0b
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-transform-list-single-values.html
@@ -0,0 +1,57 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(0px)"
+}, {
+  keyframes: ["translateX(100px)", "translateX(200px)"],
+  expected: "translateX(150px)"
+}, 'Animating a custom property of type <transform-list> containing a single value');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  keyframes: "translateX(200px)",
+  expected: "translateX(150px)"
+}, 'Animating a custom property of type <transform-list> containing a single value with a single keyframe');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  composite: "add",
+  keyframes: ["translateX(200px)", "translateX(300px)"],
+  expected: "translateX(100px) translateX(250px)"
+}, 'Animating a custom property of type <transform-list> containing a single value with additivity');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  composite: "add",
+  keyframes: "translateX(300px)",
+  expected: "translateX(100px) translateX(150px)"
+}, 'Animating a custom property of type <transform-list> containing a single value with a single keyframe and additivity');
+
+animation_test({
+  syntax: "<transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  iterationComposite: "accumulate",
+  keyframes: ["translateX(0px)", "translateX(100px)"],
+  expected: "translateX(250px)"
+}, 'Animating a custom property of type <transform-list> containing a single value with iterationComposite');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-url.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-url.html
new file mode 100644
index 0000000000..830b9e1f49
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-animation-url.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+discrete_animation_test("<url>", 'url("https://example.com/from")', 'url("https://example.com/to")');
+discrete_animation_test("<url>+", 'url("https://example.com/from1") url("https://example.com/from2")', 'url("https://example.com/to1") url("https://example.com/to2")');
+discrete_animation_test("<url>#", 'url("https://example.com/from1"), url("https://example.com/from2")', 'url("https://example.com/to1"), url("https://example.com/to2")');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-angle.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-angle.html
new file mode 100644
index 0000000000..974fc6c0db
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-angle.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<angle>",
+  from: "100deg",
+  to: "200deg",
+  expected: "150deg"
+}, 'A custom property of type <angle> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-color.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-color.html
new file mode 100644
index 0000000000..445a2a6391
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-color.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<color>",
+  from: "rgb(100, 100, 100)",
+  to: "rgb(200, 200, 200)",
+  expected: "rgb(150, 150, 150)"
+}, 'A custom property of type <color> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-custom-ident.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-custom-ident.html
new file mode 100644
index 0000000000..e22d9ce882
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-custom-ident.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+no_transition_test({
+  syntax: "<custom-ident>",
+  from: "from",
+  to: "to",
+}, 'A custom property of type <custom-ident> cannot yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-image.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-image.html
new file mode 100644
index 0000000000..b7fc68e6b8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-image.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+no_transition_test({
+  syntax: "<image>",
+  from: 'url("https://example.com/from")',
+  to: 'url("https://example.com/to")',
+}, 'A custom property of type <image> cannot yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-inherited-used-by-standard-property.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-inherited-used-by-standard-property.html
new file mode 100644
index 0000000000..0680722d4a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-inherited-used-by-standard-property.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="container">
+    <div id="target"></div>
+</div>
+<script>
+
+test(() => {
+  const customProperty = "--my-length";
+
+  CSS.registerProperty({
+    name: customProperty,
+    syntax: "<length>",
+    inherits: true,
+    initialValue: "100px"
+  });
+
+  target.style.marginLeft = `var(${customProperty})`;
+  assert_equals(getComputedStyle(target).marginLeft, "100px");
+  assert_equals(getComputedStyle(target).getPropertyValue(customProperty), "100px");
+
+  container.style.transition = `${customProperty} 1s -500ms linear`;
+  container.style.setProperty(customProperty, "200px");
+
+  assert_equals(getComputedStyle(target).marginLeft, "150px");
+}, "Running a transition an inherited CSS variable is reflected on a standard property using that variable as a value");
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-integer.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-integer.html
new file mode 100644
index 0000000000..64685fe07c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-integer.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<integer>",
+  from: "100",
+  to: "200",
+  expected: "150"
+}, 'A custom property of type <integer> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length-percentage.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length-percentage.html
new file mode 100644
index 0000000000..f1ed1dec26
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length-percentage.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<length-percentage>",
+  from: "100px",
+  to: "100%",
+  expected: "calc(50% + 50px)"
+}, 'A custom property of type <length-percentage> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length.html
new file mode 100644
index 0000000000..f5a76490cc
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-length.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<length>",
+  from: "100px",
+  to: "200px",
+  expected: "150px"
+}, 'A custom property of type <length> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-mismatched-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-mismatched-list.html
new file mode 100644
index 0000000000..7796e36a2e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-mismatched-list.html
@@ -0,0 +1,153 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+no_transition_test({
+  syntax: "<angle>#",
+  from: '100deg, 200deg',
+  to: '300deg',
+}, 'A custom property of type <angle># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<angle>+",
+  from: '100deg 200deg',
+  to: '300deg',
+}, 'A custom property of type <angle>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<color>#",
+  from: 'rgb(100, 100, 100), rgb(150, 150, 150)',
+  to: 'rgb(200, 200, 200)',
+}, 'A custom property of type <color># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<color>+",
+  from: 'rgb(100, 100, 100) rgb(150, 150, 150)',
+  to: 'rgb(200, 200, 200)',
+}, 'A custom property of type <color>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<custom-ident>#",
+  from: 'foo, bar',
+  to: 'baz',
+}, 'A custom property of type <custom-ident># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<custom-ident>+",
+  from: 'foo bar',
+  to: 'baz',
+}, 'A custom property of type <custom-ident>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<image>#",
+  from: 'url("https://example.com/foo"), url("https://example.com/bar")',
+  to: 'url("https://example.com/to")',
+}, 'A custom property of type <image># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<image>+",
+  from: 'url("https://example.com/foo") url("https://example.com/bar")',
+  to: 'url("https://example.com/to")',
+}, 'A custom property of type <image>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<integer>#",
+  from: '100, 200',
+  to: '300',
+}, 'A custom property of type <integer># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<integer>+",
+  from: '100 200',
+  to: '300',
+}, 'A custom property of type <integer>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<length-percentage>#",
+  from: '100px, 200px',
+  to: '300%',
+}, 'A custom property of type <length-percentage># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<length-percentage>+",
+  from: '100px 200px',
+  to: '300%',
+}, 'A custom property of type <length-percentage>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<length>#",
+  from: '100px, 200px',
+  to: '300px',
+}, 'A custom property of type <length># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<length>+",
+  from: '100px 200px',
+  to: '300px',
+}, 'A custom property of type <length>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<number>#",
+  from: '100, 200',
+  to: '300',
+}, 'A custom property of type <number># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<number>+",
+  from: '100 200',
+  to: '300',
+}, 'A custom property of type <number>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<percentage>#",
+  from: '100%, 200%',
+  to: '300%',
+}, 'A custom property of type <percentage># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<percentage>+",
+  from: '100% 200%',
+  to: '300%',
+}, 'A custom property of type <percentage>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<resolution>#",
+  from: '100dppx, 200dppx',
+  to: '300dppx',
+}, 'A custom property of type <resolution># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<resolution>+",
+  from: '100dppx 200dppx',
+  to: '300dppx',
+}, 'A custom property of type <resolution>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<time>#",
+  from: '100s, 200s',
+  to: '300s',
+}, 'A custom property of type <time># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<time>+",
+  from: '100s 200s',
+  to: '300s',
+}, 'A custom property of type <time>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<url>#",
+  from: 'url("https://example.com/foo"), url("https://example.com/bar")',
+  to: 'url("https://example.com/to")',
+}, 'A custom property of type <url># does not yield a CSS Transition if the lists do not contain the same number of values');
+
+no_transition_test({
+  syntax: "<url>+",
+  from: 'url("https://example.com/foo") url("https://example.com/bar")',
+  to: 'url("https://example.com/to")',
+}, 'A custom property of type <url>+ does not yield a CSS Transition if the lists do not contain the same number of values');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-non-inherited-used-by-standard-property.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-non-inherited-used-by-standard-property.html
new file mode 100644
index 0000000000..b96c28bc88
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-non-inherited-used-by-standard-property.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+test(() => {
+  const customProperty = "--my-length";
+
+  CSS.registerProperty({
+    name: customProperty,
+    syntax: "<length>",
+    inherits: false,
+    initialValue: "100px"
+  });
+
+  target.style.marginLeft = `var(${customProperty})`;
+  assert_equals(getComputedStyle(target).marginLeft, "100px");
+
+  target.style.transition = `${customProperty} 1s -500ms linear`;
+  target.style.setProperty(customProperty, "200px");
+
+  assert_equals(getComputedStyle(target).marginLeft, "150px");
+}, "Running a transition a non-inherited CSS variable is reflected on a standard property using that variable as a value");
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-number.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-number.html
new file mode 100644
index 0000000000..a96e319686
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-number.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<number>",
+  from: "100",
+  to: "200",
+  expected: "150"
+}, 'A custom property of type <number> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-percentage.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-percentage.html
new file mode 100644
index 0000000000..71d51b0e27
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-percentage.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<percentage>",
+  from: "100%",
+  to: "200%",
+  expected: "150%"
+}, 'A custom property of type <percentage> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-resolution.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-resolution.html
new file mode 100644
index 0000000000..5631910b7c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-resolution.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<resolution>",
+  from: "100dppx",
+  to: "200dppx",
+  expected: "150dppx"
+}, 'A custom property of type <resolution> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-time.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-time.html
new file mode 100644
index 0000000000..eb579b71f1
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-time.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<time>",
+  from: "100s",
+  to: "200s",
+  expected: "150s"
+}, 'A custom property of type <time> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-function.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-function.html
new file mode 100644
index 0000000000..aa491555c7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-function.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<transform-function>",
+  from: "translateX(100px)",
+  to: "translateX(200px)",
+  expected: "translateX(150px)"
+}, 'A custom property of type <transform-function> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-list.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-list.html
new file mode 100644
index 0000000000..8b7424fec5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-transform-list.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+transition_test({
+  syntax: "<transform-list>",
+  from: "translateX(100px) scale(2)",
+  to: "translateX(200px) scale(4)",
+  expected: "translateX(150px) scale(3)"
+}, 'A custom property of type <transform-list> can yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-url.html b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-url.html
new file mode 100644
index 0000000000..785db04562
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/animation/custom-property-transition-url.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../resources/utils.js"></script>
+<div id="target"></div>
+<script>
+
+no_transition_test({
+  syntax: "<url>",
+  from: 'url("https://example.com/from")',
+  to: 'url("https://example.com/to")',
+}, 'A custom property of type <url> cannot yield a CSS Transition');
+
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-animation.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-animation.html
new file mode 100644
index 0000000000..9f81ae846e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-animation.html
@@ -0,0 +1,298 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id=outer>
+  <div id=div></div>
+</div>
+<script>
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name) => {
+  with_style_node(`
+    @keyframes test {
+      from { ${name}: 100px; }
+      to { ${name}: 200px; }
+    }
+    #div { animation: test 100s -50s linear; }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '150px');
+  });
+}, '@keyframes works with @property');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name) => {
+  with_style_node(`
+    @property ${name} {
+      syntax: "<color>";
+      inherits: false;
+      initial-value: black;
+    }
+    @keyframes test {
+      from { ${name}: rgb(100, 100, 100); }
+      to { ${name}: rgb(200, 200, 200); }
+    }
+    #div { animation: test 100s -50s linear; }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(150, 150, 150)');
+  });
+}, '@keyframes picks up the latest @property in the document');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name) => {
+  // These keyframes are initially invalid for the declared custom property.
+  let animation = div.animate([
+    { [name]: 'rgb(100, 100, 100)'},
+    { [name]: 'rgb(200, 200, 200)'},
+  ], { duration: 10000, delay: -5000, easing: 'linear' });
+  let cs = getComputedStyle(div);
+  assert_equals(cs.getPropertyValue(name), '0px');
+
+  // Redeclare the property as a <color>, effectively making the existing
+  // keyframes valid.
+  with_at_property({
+    name: name,
+    syntax: '"<color>"',
+    inherits: false,
+    initialValue: 'black'
+  }, (name) => {
+    assert_equals(cs.getPropertyValue(name), 'rgb(150, 150, 150)');
+  });
+
+  animation.finish();
+}, 'Ongoing animation picks up redeclared custom property');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name) => {
+  // These keyframes are initially invalid for the declared custom property.
+  let animation = div.animate([
+    { [name]: 'rgb(100, 100, 100)'},
+    { [name]: 'rgb(200, 200, 200)'},
+  ], { duration: 10000, delay: -5000, easing: 'linear' });
+  let cs = getComputedStyle(div);
+  assert_equals(cs.getPropertyValue(name), '0px');
+
+  // Setting the keyframes to something that matches <length> makes the
+  // interpolation valid.
+  animation.effect.setKeyframes([
+    {[name]: '100px'},
+    {[name]: '200px'}
+  ]);
+  assert_equals(cs.getPropertyValue(name), '150px');
+
+  animation.finish();
+}, 'Ongoing animation matches new keyframes against the current registration');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name) => {
+  let animation = div.animate([
+    { [name]: 'initial'},
+    { [name]: '400px'},
+  ], { duration: 10000, delay: -5000, easing: 'linear' });
+  let cs = getComputedStyle(div);
+  assert_equals(cs.getPropertyValue(name), '200px');
+
+  // Change initial value.
+  with_at_property({
+    name: name,
+    syntax: '"<length>"',
+    inherits: false,
+    initialValue: '100px'
+  }, (name) => {
+    assert_equals(cs.getPropertyValue(name), '250px');
+  });
+
+  animation.finish();
+}, 'Ongoing animation picks up redeclared intial value');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name) => {
+  try {
+    document.body.style = `${name}: 100px`;
+    // Note that 'inherit' here refers to #outer, which has the initial
+    // value. (#outer did not inherit from body, since the property is not
+    // yet declared as inherited).
+    let animation = div.animate([
+      { [name]: 'inherit'},
+      { [name]: '400px'},
+    ], { duration: 10000, delay: -5000, easing: 'linear' });
+    let cs = getComputedStyle(div);
+    assert_equals(cs.getPropertyValue(name), '200px');
+
+    // Change inherits to 'true'. The value should now propagate from body
+    // to #outer.
+    with_at_property({
+      name: name,
+      syntax: '"<length>"',
+      inherits: true,
+      initialValue: '0px'
+    }, (name) => {
+      assert_equals(cs.getPropertyValue(name), '250px');
+    });
+
+    animation.finish();
+  } finally {
+    document.body.style = '';
+  }
+}, 'Ongoing animation picks up redeclared inherits flag');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name) => {
+  try {
+    outer.style = `${name}: 100px`;
+    // 'unset' should take the initial value (not the value from #outer), since
+    // the property is not declared as inherited.
+    let animation = div.animate([
+      { [name]: 'unset'},
+      { [name]: '400px'},
+    ], { duration: 10000, delay: -5000, easing: 'linear' });
+    let cs = getComputedStyle(div);
+    assert_equals(cs.getPropertyValue(name), '200px');
+
+    // Change inherits to 'true'. 'unset' now refers to #outer's value.
+    with_at_property({
+      name: name,
+      syntax: '"<length>"',
+      inherits: true,
+      initialValue: '0px'
+    }, (name) => {
+      assert_equals(cs.getPropertyValue(name), '250px');
+    });
+
+    animation.finish();
+  } finally {
+    outer.style = '';
+  }
+}, 'Ongoing animation picks up redeclared meaning of \'unset\'');
+
+test_with_at_property({
+  syntax: '"<color>"',
+  inherits: false,
+  initialValue: 'red'
+}, (name) => {
+  try {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(255, 0, 0)');
+    div.style = `transition: ${name} steps(2, start) 100s; ${name}: blue`;
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(128, 0, 128)');
+  } finally {
+    div.style = '';
+  }
+}, 'Transitioning from initial value');
+
+test_with_at_property({
+  syntax: '"<color>"',
+  inherits: false,
+  initialValue: 'red'
+}, (name) => {
+  try {
+    div.style = `${name}: blue;`;
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(0, 0, 255)');
+    div.style = `transition: ${name} steps(2, start) 100s; ${name}: green`;
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(0, 64, 128)');
+  } finally {
+    div.style = '';
+  }
+}, 'Transitioning from specified value');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '100px'
+}, (name) => {
+    with_style_node(`div { transition: ${name} steps(2, start) 100s; }`, () => {
+      assert_equals(getComputedStyle(div).getPropertyValue(name), '100px');
+      // Re-declaring the property with a different initial value effectively
+      // means the computed value has changed. This means we should transition
+      // from the old initial value to the new initial value.
+      with_at_property({
+        name: name,
+        syntax: '"<length>"',
+        inherits: false,
+        initialValue: '200px'
+      }, () => {
+        assert_equals(getComputedStyle(div).getPropertyValue(name), '150px');
+      });
+    });
+}, 'Transition triggered by initial value change');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '100px'
+}, (name) => {
+    with_style_node(`div { transition: ${name} steps(2, start) 100s; }`, () => {
+      assert_equals(getComputedStyle(div).getPropertyValue(name), '100px');
+      with_at_property({
+        name: name,
+        syntax: '"<color>"',
+        inherits: false,
+        initialValue: 'green'
+      }, () => {
+        assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(0, 128, 0)');
+      });
+    });
+}, 'No transition when changing types');
+
+test(() => {
+  let name = generate_name();
+  with_style_node(`div { ${name}: 100px; transition: ${name} steps(2, start) 100s; }`, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '100px');
+
+    let style1 = document.createElement('style');
+    style1.textContent = `
+      @property ${name} {
+        syntax: "<length>";
+        inherits: false;
+        initial-value: 200px;
+      }
+    `;
+
+    let style2 = document.createElement('style');
+    style2.textContent = `div { ${name}: 400px; }`;
+
+    try {
+      // Register the property:
+      document.body.append(style1);
+      // The token sequence ' 100px' is now interpreted as a length '100px'.
+      assert_equals(getComputedStyle(div).getPropertyValue(name), '100px');
+
+      // Change the computed value:
+      document.body.append(style2);
+      // This should cause an interpolation between 100px and 400px:
+      assert_equals(getComputedStyle(div).getPropertyValue(name), '250px');
+
+      // In the middle of the transition above, remove the @property rule
+      // (making the computed value a token sequence again). We should snap
+      // to the new token sequence.
+      style1.remove();
+      assert_equals(getComputedStyle(div).getPropertyValue(name), '400px');
+    } finally {
+      style1.remove();
+      style2.remove();
+    }
+  });
+}, 'No transition when removing @property rule');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-cssom.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-cssom.html
new file mode 100644
index 0000000000..f7981f245a
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-cssom.html
@@ -0,0 +1,197 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#cssom">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  @property --valid {
+    syntax: "<color> | none";
+    inherits: false;
+    initial-value: red;
+  }
+  @property --valid-reverse {
+    initial-value: 0px;
+    inherits: true;
+    syntax: "<length>";
+  }
+  @property --valid-universal {
+    syntax: "*";
+    inherits: false;
+  }
+  @property --valid-whitespace {
+    syntax: " <color>+ ";
+    inherits: false;
+    initial-value: red, blue;
+  }
+  @property --vALId {
+    syntax: "<color> | none";
+    inherits: false;
+    initial-value: red;
+  }
+  @property --no-descriptors {
+
+  }
+  @property --no-syntax {
+    inherits: false;
+    initial-value: red;
+  }
+  @property --no-inherits {
+    syntax: "<color> | none";
+    initial-value: red;
+  }
+  @property --no-initial-value {
+    syntax: "<color> | none";
+    inherits: false;
+  }
+  @property --syntax-only {
+    syntax: "<color> | none";
+  }
+  @property --inherits-only {
+    inherits: true;
+  }
+  @property --initial-value-only {
+    initial-value: red;
+  }
+  /* U+0009 CHARACTER TABULATION */
+  @property --tab\9 tab {  }
+</style>
+<script>
+
+function find_at_property_rule(name) {
+  for (let rule of document.styleSheets[0].cssRules) {
+    if (rule.constructor.name != "CSSPropertyRule")
+      continue;
+    if (rule.name == name)
+      return rule;
+  }
+  return null;
+}
+
+function test_css_text(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.cssText, expected);
+  }, `Rule for ${name} has expected cssText`);
+}
+
+function test_name(name) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.name, name);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.name`);
+}
+
+function test_syntax(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.syntax, expected);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.syntax`);
+}
+
+function test_inherits(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.inherits, expected);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.inherits`);
+}
+
+function test_initial_value(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.initialValue, expected);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.initialValue`);
+}
+
+// CSSPropertyRule.cssText
+
+test_css_text('--valid', '@property --valid { syntax: "<color> | none"; inherits: false; initial-value: red; }');
+test_css_text('--valid-reverse', '@property --valid-reverse { syntax: "<length>"; inherits: true; initial-value: 0px; }');
+test_css_text('--valid-universal', '@property --valid-universal { syntax: "*"; inherits: false; }');
+test_css_text('--valid-whitespace', '@property --valid-whitespace { syntax: " <color>+ "; inherits: false; initial-value: red, blue; }');
+test_css_text('--vALId', '@property --vALId { syntax: "<color> | none"; inherits: false; initial-value: red; }');
+
+test_css_text('--no-descriptors', '@property --no-descriptors { }');
+test_css_text('--no-syntax', '@property --no-syntax { inherits: false; initial-value: red; }');
+test_css_text('--no-inherits', '@property --no-inherits { syntax: "<color> | none"; initial-value: red; }');
+test_css_text('--no-initial-value', '@property --no-initial-value { syntax: "<color> | none"; inherits: false; }');
+test_css_text('--syntax-only', '@property --syntax-only { syntax: "<color> | none"; }');
+test_css_text('--inherits-only', '@property --inherits-only { inherits: true; }');
+test_css_text('--initial-value-only', '@property --initial-value-only { initial-value: red; }');
+test_css_text('--tab\ttab', '@property --tab\\9 tab { }');
+
+// CSSRule.type
+
+test(() => {
+  let rule = find_at_property_rule('--valid');
+  assert_equals(rule.type, 0);
+}, 'CSSRule.type returns 0');
+
+// CSSPropertyRule.name
+
+test_name('--valid');
+test_name('--valid-reverse');
+test_name('--valid-universal');
+test_name('--valid-whitespace');
+test_name('--vALId');
+
+test_name('--no-descriptors');
+test_name('--no-syntax');
+test_name('--no-inherits');
+test_name('--no-initial-value');
+test_name('--syntax-only');
+test_name('--inherits-only');
+test_name('--initial-value-only');
+
+// CSSPropertyRule.syntax
+
+test_syntax('--valid', '<color> | none');
+test_syntax('--valid-reverse', '<length>');
+test_syntax('--valid-universal', '*');
+test_syntax('--valid-whitespace', ' <color>+ ');
+test_syntax('--vALId', '<color> | none');
+
+test_syntax('--no-descriptors', '');
+test_syntax('--no-syntax', '');
+test_syntax('--no-inherits', '<color> | none');
+test_syntax('--no-initial-value', '<color> | none');
+test_syntax('--syntax-only', '<color> | none');
+test_syntax('--inherits-only', '');
+test_syntax('--initial-value-only', '');
+
+// CSSPropertyRule.inherits
+
+test_inherits('--valid', false);
+test_inherits('--valid-reverse', true);
+test_inherits('--valid-universal', false);
+test_inherits('--valid-whitespace', false);
+test_inherits('--vALId', false);
+
+test_inherits('--no-descriptors', false);
+test_inherits('--no-syntax', false);
+test_inherits('--no-inherits', false);
+test_inherits('--no-initial-value', false);
+test_inherits('--syntax-only', false);
+test_inherits('--inherits-only', true);
+test_inherits('--initial-value-only', false);
+
+// CSSPropertyRule.initialValue
+
+test_initial_value('--valid', 'red');
+test_initial_value('--valid-reverse', '0px');
+test_initial_value('--valid-universal', null);
+test_initial_value('--valid-whitespace', 'red, blue');
+test_initial_value('--vALId', 'red');
+
+test_initial_value('--no-descriptors', null);
+test_initial_value('--no-syntax', 'red');
+test_initial_value('--no-inherits', 'red');
+test_initial_value('--no-initial-value', null);
+test_initial_value('--syntax-only', null);
+test_initial_value('--inherits-only', null);
+test_initial_value('--initial-value-only', 'red');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-non-matching-media-crash.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-non-matching-media-crash.html
new file mode 100644
index 0000000000..c5b894f349
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-non-matching-media-crash.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<html class="test-wait">
+  <link rel="help" href="https://crbug.com/1085994">
+  <style id="style">
+    @property --x {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 0px;
+    }
+  </style>
+  <script>
+      document.documentElement.offsetTop;
+      style.setAttribute('media', 'braille');
+      document.documentElement.className = '';
+  </script>
+  <p>
+    PASS if no crash
+  </p>
+</html>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-shadow.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-shadow.html
new file mode 100644
index 0000000000..db282d10b0
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-shadow.html
@@ -0,0 +1,46 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#at-property-rule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<style>
+  @property --x {
+    syntax: "<length>";
+    inherits: false;
+    initial-value: 0px;
+  }
+  #outside {
+    --x: calc(1px + 1px);
+    --y: calc(1px + 1px);
+  }
+</style>
+<template id=template>
+  <style>
+    /* This rule should have no effect */
+    @property --y {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 0px;
+    }
+    #inside {
+      --x: calc(1px + 1px);
+      --y: calc(1px + 1px);
+    }
+  </style>
+  <div id=inside></div>
+</template>
+<div id=host></div>
+<div id=outside></div>
+<script>
+
+test(() => {
+  let root = host.attachShadow({ mode: 'open' });
+  root.append(template.content.cloneNode(true));
+  let inside = root.querySelector('#inside');
+  assert_equals(getComputedStyle(outside).getPropertyValue('--x'), '2px');
+  assert_equals(getComputedStyle(outside).getPropertyValue('--y'), 'calc(1px + 1px)');
+  assert_equals(getComputedStyle(inside).getPropertyValue('--x'), '2px');
+  assert_equals(getComputedStyle(inside).getPropertyValue('--y'), 'calc(1px + 1px)');
+}, '@property rules in shadow trees should have no effect');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-stylesheets.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-stylesheets.html
new file mode 100644
index 0000000000..a7f3ea3948
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-stylesheets.html
@@ -0,0 +1,106 @@
+<!DOCTYPE html>
+<title>Verify that the correct registration is selected for mutated stylesheets</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#determining-registration">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id=div></div>
+<script>
+
+test(() => {
+  with_at_property({
+    syntax: '"<length>"',
+    inherits: false,
+    initialValue: '1px'
+  }, (name) => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+  });
+}, '@property detected when stylesheet appears');
+
+test(() => {
+  let name = generate_name();
+  with_at_property({
+    name: name,
+    syntax: '"<length>"',
+    inherits: false,
+    initialValue: '1px'
+  }, (name) => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+  });
+  assert_equals(getComputedStyle(div).getPropertyValue(name), '');
+}, '@property removal detected when last @property rule disappears');
+
+test(() => {
+  with_at_property({
+    syntax: '"<length>"',
+    inherits: false,
+    initialValue: '1px'
+  }, (name1) => {
+    with_at_property({
+      syntax: '"<length>"',
+      inherits: false,
+      initialValue: '2px'
+    }, (name2) => {
+      assert_equals(getComputedStyle(div).getPropertyValue(name2), '2px');
+    });
+  });
+}, '@property detected in second stylesheet');
+
+test(() => {
+  let name2 = generate_name();
+  with_at_property({
+    syntax: '"<length>"',
+    inherits: false,
+    initialValue: '1px'
+  }, (name1) => {
+    with_at_property({
+      name2: name2,
+      syntax: '"<length>"',
+      inherits: false,
+      initialValue: '2px'
+    }, (name2) => {
+      assert_equals(getComputedStyle(div).getPropertyValue(name2), '2px');
+    });
+    assert_equals(getComputedStyle(div).getPropertyValue(name2), '');
+  });
+}, '@property removal detected with removal of second stylesheet');
+
+test(() => {
+  let name1 = generate_name();
+  let name2 = generate_name();
+
+  let sheet1 = `
+    @property ${name1} {
+      inherits: false;
+      syntax: "<length>";
+      initial-value: 1px;
+    }
+  `;
+  let sheet2 = `
+    @property ${name2} {
+      inherits: false;
+      syntax: "<length>";
+      initial-value: 2px;
+    }
+  `;
+
+  let node1 = document.createElement('style');
+  let node2 = document.createElement('style');
+
+  node1.textContent = sheet1;
+  node2.textContent = sheet2;
+
+  try {
+    document.body.append(node1, node2);
+    assert_equals(getComputedStyle(div).getPropertyValue(name1), '1px');
+    assert_equals(getComputedStyle(div).getPropertyValue(name2), '2px');
+    node1.remove();
+    assert_equals(getComputedStyle(div).getPropertyValue(name1), '');
+    assert_equals(getComputedStyle(div).getPropertyValue(name2), '2px');
+  } finally {
+    node1.remove();
+    node2.remove();
+  }
+}, '@property removal detected with removal of first stylesheet');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-typedom.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-typedom.html
new file mode 100644
index 0000000000..04c40c8be0
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-typedom.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#css-style-value-reification">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id=div></div>
+<script>
+
+test(() => {
+  let name = generate_name();
+  with_style_node(`div { ${name}: 100px; }`, () => {
+    // Before registering the property, ${name} should reify as a
+    // a token sequence.
+    assert_equals(div.computedStyleMap().get(name).constructor.name, 'CSSUnparsedValue');
+    assert_equals(div.computedStyleMap().get(name).toString(), ' 100px');
+
+    with_at_property({
+      name: name,
+      syntax: '"<length>"',
+      inherits: false,
+      initialValue: '0px'
+    }, () => {
+      // After registering, it should reify as a <length>.
+      assert_equals(div.computedStyleMap().get(name).constructor.name, 'CSSUnitValue');
+      assert_equals(div.computedStyleMap().get(name).value, 100);
+      assert_equals(div.computedStyleMap().get(name).unit, 'px');
+    });
+
+    // After @property is removed, the computed value is once again a token
+    // sequence.
+    assert_equals(div.computedStyleMap().get(name).constructor.name, 'CSSUnparsedValue');
+    assert_equals(div.computedStyleMap().get(name).toString(), ' 100px');
+  });
+}, 'Properties declared with @property reify correctly');
+
+test(() => {
+  let name = generate_name();
+  // 0 is valid as a both <length> and <integer>, which reify differently.
+  with_style_node(`div { ${name}: 0; }`, () => {
+    with_at_property({
+      name: name,
+      syntax: '"<length>"',
+      inherits: false,
+      initialValue: '1000px'
+    }, () => {
+      assert_equals(div.computedStyleMap().get(name).constructor.name, 'CSSUnitValue');
+      assert_equals(div.computedStyleMap().get(name).value, 0);
+      assert_equals(div.computedStyleMap().get(name).unit, 'px');
+
+      with_at_property({
+        name: name,
+        syntax: '"<integer>"',
+        inherits: false,
+        initialValue: '1000'
+      }, () => {
+        assert_equals(div.computedStyleMap().get(name).constructor.name, 'CSSUnitValue');
+        assert_equals(div.computedStyleMap().get(name).value, 0);
+        assert_equals(div.computedStyleMap().get(name).unit, 'number');
+      });
+    });
+  });
+}, 'Re-declaring a property with a different type affects reification');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units-dynamic.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units-dynamic.html
new file mode 100644
index 0000000000..68adff6887
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units-dynamic.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<title>@property: viewport units in initial value (dynamic)</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#initial-value-descriptor" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  iframe {
+    width: 400px;
+    height: 200px;
+  }
+</style>
+<iframe id=iframe srcdoc="
+  <style>
+    @property --10vw { syntax: '<length>'; inherits: true; initial-value: 10vw}
+    @property --10vh { syntax: '<length>'; inherits: true; initial-value: 10vh}
+    div {
+      background: green;
+      width: var(--10vw);
+      height: var(--10vh);
+    }
+  </style>
+  <div style='width:10vw'></div>
+"></iframe>
+<script>
+  iframe.offsetTop;
+
+  function waitForLoad(w) {
+    return new Promise(resolve => w.addEventListener('load', resolve));
+  }
+
+  promise_test(async (t) => {
+    await waitForLoad(window);
+    let element = iframe.contentDocument.querySelector('div');
+    assert_equals(getComputedStyle(element).getPropertyValue('--10vw'), '40px');
+    assert_equals(getComputedStyle(element).getPropertyValue('--10vh'), '20px');
+
+    iframe.style.width = '100px';
+    assert_equals(getComputedStyle(element).getPropertyValue('--10vw'), '10px');
+    assert_equals(getComputedStyle(element).getPropertyValue('--10vh'), '20px');
+  });
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units.html b/testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units.html
new file mode 100644
index 0000000000..51520c2a70
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property-viewport-units.html
@@ -0,0 +1,91 @@
+<!DOCTYPE html>
+<title>@property: viewport units in initial value</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#initial-value-descriptor" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  iframe {
+    width: 400px;
+    height: 200px;
+  }
+</style>
+<iframe id=iframe srcdoc="
+  <style>
+    @property --10vw { syntax: '<length>'; inherits: true; initial-value: 10vw}
+    @property --10vh { syntax: '<length>'; inherits: true; initial-value: 10vh}
+    @property --10vi { syntax: '<length>'; inherits: true; initial-value: 10vi}
+    @property --10vb { syntax: '<length>'; inherits: true; initial-value: 10vb}
+    @property --10vmin { syntax: '<length>'; inherits: true; initial-value: 10vmin}
+    @property --10vmax { syntax: '<length>'; inherits: true; initial-value: 10vmax}
+
+    @property --10svw { syntax: '<length>'; inherits: true; initial-value: 10svw}
+    @property --10svh { syntax: '<length>'; inherits: true; initial-value: 10svh}
+    @property --10svi { syntax: '<length>'; inherits: true; initial-value: 10svi}
+    @property --10svb { syntax: '<length>'; inherits: true; initial-value: 10svb}
+    @property --10svmin { syntax: '<length>'; inherits: true; initial-value: 10svmin}
+    @property --10svmax { syntax: '<length>'; inherits: true; initial-value: 10svmax}
+
+    @property --10lvw { syntax: '<length>'; inherits: true; initial-value: 10lvw}
+    @property --10lvh { syntax: '<length>'; inherits: true; initial-value: 10lvh}
+    @property --10lvi { syntax: '<length>'; inherits: true; initial-value: 10lvi}
+    @property --10lvb { syntax: '<length>'; inherits: true; initial-value: 10lvb}
+    @property --10lvmin { syntax: '<length>'; inherits: true; initial-value: 10lvmin}
+    @property --10lvmax { syntax: '<length>'; inherits: true; initial-value: 10lvmax}
+
+    @property --10dvw { syntax: '<length>'; inherits: true; initial-value: 10dvw}
+    @property --10dvh { syntax: '<length>'; inherits: true; initial-value: 10dvh}
+    @property --10dvi { syntax: '<length>'; inherits: true; initial-value: 10dvi}
+    @property --10dvb { syntax: '<length>'; inherits: true; initial-value: 10dvb}
+    @property --10dvmin { syntax: '<length>'; inherits: true; initial-value: 10dvmin}
+    @property --10dvmax { syntax: '<length>'; inherits: true; initial-value: 10dvmax}
+  </style>
+  <div></div>
+"></iframe>
+<script>
+  iframe.offsetTop;
+
+  function waitForLoad(w) {
+    return new Promise(resolve => {
+      if (w.document.readyState == 'complete')
+        resolve();
+      else
+        w.addEventListener('load', resolve)
+    });
+  }
+
+  function test_unit(element, actual, expected) {
+    promise_test(async (t) => {
+      await waitForLoad(window);
+      let element = iframe.contentDocument.querySelector('div');
+      assert_equals(getComputedStyle(element).getPropertyValue(`--${actual}`), expected);
+    },`${actual} is ${expected}`);
+  }
+
+  test_unit(iframe, '10vw', '40px');
+  test_unit(iframe, '10vh', '20px');
+  test_unit(iframe, '10vi', '40px');
+  test_unit(iframe, '10vb', '20px');
+  test_unit(iframe, '10vmin', '20px');
+  test_unit(iframe, '10vmax', '40px');
+
+  test_unit(iframe, '10svw', '40px');
+  test_unit(iframe, '10svh', '20px');
+  test_unit(iframe, '10svi', '40px');
+  test_unit(iframe, '10svb', '20px');
+  test_unit(iframe, '10svmin', '20px');
+  test_unit(iframe, '10svmax', '40px');
+
+  test_unit(iframe, '10lvw', '40px');
+  test_unit(iframe, '10lvh', '20px');
+  test_unit(iframe, '10lvi', '40px');
+  test_unit(iframe, '10lvb', '20px');
+  test_unit(iframe, '10lvmin', '20px');
+  test_unit(iframe, '10lvmax', '40px');
+
+  test_unit(iframe, '10dvw', '40px');
+  test_unit(iframe, '10dvh', '20px');
+  test_unit(iframe, '10dvi', '40px');
+  test_unit(iframe, '10dvb', '20px');
+  test_unit(iframe, '10dvmin', '20px');
+  test_unit(iframe, '10dvmax', '40px');
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/at-property.html b/testing/web-platform/tests/css/css-properties-values-api/at-property.html
new file mode 100644
index 0000000000..d996091dc2
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/at-property.html
@@ -0,0 +1,224 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#at-property-rule">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id="outer">
+  <div id="target"></div>
+</div>
+<script>
+
+// Parsing:
+
+let uppercase_first = (x) => x.charAt(0).toUpperCase() + x.slice(1);
+let to_camel_case = (x) => x.split('-')[0] + x.split('-').slice(1).map(uppercase_first).join('');
+
+function get_cssom_descriptor_value(rule, descriptor) {
+  switch (descriptor) {
+    case 'syntax':
+      return rule.syntax;
+    case 'inherits':
+      return rule.inherits;
+    case 'initial-value':
+      return rule.initialValue;
+    default:
+      assert_true(false, 'Should not reach here');
+      return null;
+  }
+}
+
+// Test that for the given descriptor (e.g. 'syntax'), the specified value
+// will yield the expected_value when observed using CSSOM. If the expected_value
+// is omitted, it is the same as the specified value.
+function test_descriptor(descriptor, specified_value, expected_value) {
+  let camel = to_camel_case(descriptor);
+  if (typeof(expected_value) === 'undefined')
+    expected_value = specified_value;
+  test_with_at_property({ [camel]: specified_value }, (name, rule) => {
+    assert_equals(get_cssom_descriptor_value(rule, descriptor), expected_value);
+  }, `Attribute '${descriptor}' returns expected value for [${specified_value}]`);
+}
+
+// syntax
+test_descriptor('syntax', '"<color>"', '<color>');
+test_descriptor('syntax', '"<color> | none"', '<color> | none');
+test_descriptor('syntax', '"<color># | <image> | none"', '<color># | <image> | none');
+test_descriptor('syntax', '"foo | bar | baz"', 'foo | bar | baz');
+test_descriptor('syntax', '"*"', '*');
+test_descriptor('syntax', '"notasyntax"', 'notasyntax');
+
+test_descriptor('syntax', 'red', '');
+test_descriptor('syntax', 'rgb(255, 0, 0)', '');
+test_descriptor('syntax', '<color>', '');
+test_descriptor('syntax', 'foo | bar', '');
+
+// initial-value
+test_descriptor('initial-value', '10px');
+test_descriptor('initial-value', 'rgb(1, 2, 3)');
+test_descriptor('initial-value', 'red');
+test_descriptor('initial-value', 'foo');
+test_descriptor('initial-value', 'if(){}');
+test_descriptor('initial-value', 'var(--x)');
+
+// inherits
+test_descriptor('inherits', 'true', true);
+test_descriptor('inherits', 'false', false);
+
+test_descriptor('inherits', 'none', false);
+test_descriptor('inherits', '0', false);
+test_descriptor('inherits', '1', false);
+test_descriptor('inherits', '"true"', false);
+test_descriptor('inherits', '"false"', false);
+test_descriptor('inherits', 'calc(0)', false);
+
+test_with_style_node('@property foo { }', (node) => {
+  assert_equals(node.sheet.rules.length, 0);
+}, 'Invalid property name does not parse [foo]');
+
+test_with_style_node('@property -foo { }', (node) => {
+  assert_equals(node.sheet.rules.length, 0);
+}, 'Invalid property name does not parse [-foo]');
+
+// Applying @property rules
+
+function test_applied(syntax, initial, inherits, expected) {
+  test_with_at_property({
+    syntax: `"${syntax}"`,
+    initialValue: initial,
+    inherits: inherits
+  }, (name, rule) => {
+    let actual = getComputedStyle(target).getPropertyValue(name);
+    assert_equals(actual, expected);
+  }, `Rule applied [${syntax}, ${initial}, ${inherits}]`);
+}
+
+function test_not_applied(syntax, initial, inherits) {
+  test_with_at_property({
+    syntax: `"${syntax}"`,
+    initialValue: initial,
+    inherits: inherits
+  }, (name, rule) => {
+    let actual = getComputedStyle(target).getPropertyValue(name);
+    assert_equals(actual, '');
+  }, `Rule not applied [${syntax}, ${initial}, ${inherits}]`);
+}
+
+// syntax, initialValue, inherits, expected
+test_applied('*', 'if(){}', false, 'if(){}');
+test_applied('<angle>', '42deg', false, '42deg');
+test_applied('<angle>', '1turn', false, '360deg');
+test_applied('<color>', 'green', false, 'rgb(0, 128, 0)');
+test_applied('<color>', 'rgb(1, 2, 3)', false, 'rgb(1, 2, 3)');
+test_applied('<image>', 'url("http://a/")', false, 'url("http://a/")');
+test_applied('<integer>', '5', false, '5');
+test_applied('<length-percentage>', '10px', false, '10px');
+test_applied('<length-percentage>', '10%', false, '10%');
+test_applied('<length-percentage>', 'calc(10% + 10px)', false, 'calc(10% + 10px)');
+test_applied('<length>', '10px', false, '10px');
+test_applied('<number>', '2.5', false, '2.5');
+test_applied('<percentage>', '10%', false, '10%');
+test_applied('<resolution>', '50dppx', false, '50dppx');
+test_applied('<resolution>', '96dpi', false, '1dppx');
+test_applied('<time>', '10s', false, '10s');
+test_applied('<time>', '1000ms', false, '1s');
+test_applied('<transform-function>', 'rotateX(0deg)', false, 'rotateX(0deg)');
+test_applied('<transform-list>', 'rotateX(0deg)', false, 'rotateX(0deg)');
+test_applied('<transform-list>', 'rotateX(0deg) translateX(10px)', false, 'rotateX(0deg) translateX(10px)');
+test_applied('<url>', 'url("http://a/")', false, 'url("http://a/")');
+
+// inherits: true/false
+test_applied('<color>', 'tomato', false, 'rgb(255, 99, 71)');
+test_applied('<color>', 'tomato', true, 'rgb(255, 99, 71)');
+
+test_with_at_property({ syntax: '"*"', inherits: true }, (name, rule) => {
+  try {
+    outer.style.setProperty(name, 'foo');
+    let actual = getComputedStyle(target).getPropertyValue(name);
+    assert_equals(actual, 'foo');
+  } finally {
+    outer.style = '';
+  }
+}, 'Rule applied for "*", even with no initial value');
+
+test_not_applied(undefined, 'green', false);
+test_not_applied('<color>', undefined, false);
+test_not_applied('<color>', 'green', undefined);
+test_not_applied('<gandalf>', 'grey', false);
+test_not_applied('gandalf', 'grey', false);
+test_not_applied('<color>', 'notacolor', false);
+test_not_applied('<length>', '10em', false);
+
+// Inheritance
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '0px'
+}, (name, rule) => {
+  try {
+    outer.style = `${name}: 40px`;
+    assert_equals(getComputedStyle(outer).getPropertyValue(name), '40px');
+    assert_equals(getComputedStyle(target).getPropertyValue(name), '0px');
+  } finally {
+    outer.style = '';
+  }
+}, 'Non-inherited properties do not inherit');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: true,
+  initialValue: '0px'
+}, (name, rule) => {
+  try {
+    outer.style = `${name}: 40px`;
+    assert_equals(getComputedStyle(outer).getPropertyValue(name), '40px');
+    assert_equals(getComputedStyle(target).getPropertyValue(name), '40px');
+  } finally {
+    outer.style = '';
+  }
+}, 'Inherited properties inherit');
+
+// Initial values
+
+test_with_at_property({
+  syntax: '"<color>"',
+  inherits: true,
+  initialValue: 'green'
+}, (name, rule) => {
+  try {
+    target.style = `--x:var(${name})`;
+    assert_equals(getComputedStyle(target).getPropertyValue(name), 'rgb(0, 128, 0)');
+  } finally {
+    target.style = '';
+  }
+}, 'Initial values substituted as computed value');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: undefined
+}, (name, rule) => {
+  try {
+    target.style = `${name}: calc(1px + 1px);`;
+    assert_equals(getComputedStyle(target).getPropertyValue(name), 'calc(1px + 1px)');
+  } finally {
+    target.style = '';
+  }
+}, 'Non-universal registration are invalid without an initial value');
+
+test_with_at_property({
+  syntax: '"*"',
+  inherits: false,
+  initialValue: undefined
+}, (name, rule) => {
+  try {
+    // If the registration suceeded, ${name} does *not* inherit, and hence
+    // the computed value on 'target' should be empty.
+    outer.style = `${name}: calc(1px + 1px);`;
+    assert_equals(getComputedStyle(target).getPropertyValue(name), '');
+  } finally {
+    outer.style = '';
+  }
+}, 'Initial value may be omitted for universal registration');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/conditional-rules.html b/testing/web-platform/tests/css/css-properties-values-api/conditional-rules.html
new file mode 100644
index 0000000000..0bff879856
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/conditional-rules.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#conditional-rules">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<script>
+  CSS.registerProperty({
+    name: '--length',
+    syntax: '<length>',
+    initialValue: '0px',
+    inherits: false
+  });
+</script>
+
+<style>
+  #target { color: red; }
+  @supports(--length: green) {
+    #target { color: rgb(1, 2, 3); }
+  }
+</style>
+
+<div id=target></div>
+
+<script>
+
+test(function() {
+  let cs = getComputedStyle(target);
+  assert_equals(cs.getPropertyValue('color'), 'rgb(1, 2, 3)');
+}, '@supports should ignore registered syntax');
+
+test(function() {
+  assert_true(CSS.supports('--length: red'));
+  assert_true(CSS.supports('--length: 10px'));
+  assert_true(CSS.supports('--length: anything, really'));
+}, 'CSS.supports(conditionText) should ignore registered syntax');
+
+test(function() {
+  assert_true(CSS.supports('--length', 'red'));
+  assert_true(CSS.supports('--length', '10px'));
+  assert_true(CSS.supports('--length', ' anything, really'));
+}, 'CSS.supports(property, value) should ignore registered syntax');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/crashtests/initial-in-audio-crash.html b/testing/web-platform/tests/css/css-properties-values-api/crashtests/initial-in-audio-crash.html
new file mode 100644
index 0000000000..dfc2b850a5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/crashtests/initial-in-audio-crash.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<link rel="help" href="https://crbug.com/1356699">
+<audio>
+  <div>
+    <div id=test1></div>
+  </div>
+</audio>
+<script>
+getComputedStyle(test1).getPropertyValue('--a');
+CSS.registerProperty({name: '--a', syntax: '<length>', initialValue: '2px', inherits: false});
+test1.style.setProperty('--a', 'initial');
+getComputedStyle(test1).getPropertyValue('--a');
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/determine-registration.html b/testing/web-platform/tests/css/css-properties-values-api/determine-registration.html
new file mode 100644
index 0000000000..20cddc48ad
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/determine-registration.html
@@ -0,0 +1,232 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#determining-registration">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id=outer>
+  <div id=div></div>
+</div>
+<script>
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '1px'
+}, (name) => {
+  assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+}, '@property determines the registration when uncontested');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '2px'
+}, (name) => {
+  CSS.registerProperty({
+    name: name,
+    syntax: '<color>',
+    inherits: false,
+    initialValue: 'green'
+  });
+  assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(0, 128, 0)');
+}, 'CSS.registerProperty wins over @property');
+
+test_with_at_property({
+  syntax: '"<length>"',
+  inherits: false,
+  initialValue: '3px'
+}, (name1) => {
+  with_at_property({
+    name: name1,
+    syntax: '"<integer>"',
+    inherits: false,
+    initialValue: '6'
+  }, (name2) => {
+    assert_equals(name1, name2);
+    assert_equals(getComputedStyle(div).getPropertyValue(name2), '6');
+  });
+}, '@property later in document order wins');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`
+    @property ${name} {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 4px;
+    }
+
+    @property ${name} {
+      syntax: "<color>";
+      inherits: false;
+      initial-value: red;
+    }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(255, 0, 0)');
+  });
+}, '@property later in stylesheet wins');
+
+test(() => {
+  let name = generate_name();
+  CSS.registerProperty({
+    name: name,
+    syntax: '<color>',
+    inherits: false,
+    initialValue: 'green'
+  });
+  assert_equals(getComputedStyle(div).getPropertyValue(name), 'rgb(0, 128, 0)');
+}, 'CSS.registerProperty determines the registration when uncontested');
+
+test(() => {
+  let name = generate_name();
+
+  // ${name} is initially not registered, hence has no initial value.
+  assert_equals(getComputedStyle(div).getPropertyValue(name), '');
+
+  with_at_property({
+    name: name,
+    syntax: '"<length>"',
+    inherits: false,
+    initialValue: '10px'
+  }, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '10px');
+  });
+
+  // When the style node is removed, ${name} should be unregistered again.
+  assert_equals(getComputedStyle(div).getPropertyValue(name), '');
+}, '@property registrations are cleared when rule removed');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`div { ${name}: calc(1px + 1px); }`, () => {
+    // ${name} should be a token sequence at this point.
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'calc(1px + 1px)');
+
+    with_at_property({
+      name: name,
+      syntax: '"<length>"',
+      inherits: false,
+      initialValue: '0px'
+    }, () => {
+      // ${name} is now a <length>, hence the calc() should be simplified.
+      assert_equals(getComputedStyle(div).getPropertyValue(name), '2px');
+    });
+
+    // ${name} should be a token sequence again.
+    assert_equals(getComputedStyle(div).getPropertyValue(name), 'calc(1px + 1px)');
+  });
+}, 'Computed value becomes token sequence when @property is removed');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`#outer { ${name}: 10px; }`, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '10px');
+
+    with_at_property({
+      name: name,
+      syntax: '"<length>"',
+      inherits: false,
+      initialValue: '0px'
+    }, () => {
+      // ${name} is no longer inherited
+      assert_equals(getComputedStyle(div).getPropertyValue(name), '0px');
+    });
+
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '10px');
+  });
+}, 'Inherited status is reflected in computed styles when @property is removed');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`
+    @property ${name} {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 1px;
+    }
+
+    @property ${name} {
+      inherits: false;
+      initial-value: green;
+    }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+  });
+}, 'Invalid @property rule (missing syntax) does not overwrite previous valid rule');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`
+    @property ${name} {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 1px;
+    }
+
+    @property ${name} {
+      syntax: "<color>";
+      initial-value: green;
+    }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+  });
+}, 'Invalid @property rule (missing inherits descriptor) does not overwrite previous valid rule');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`
+    @property ${name} {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 1px;
+    }
+
+    @property ${name} {
+      syntax: "<color>";
+      inherits: false;
+    }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+  });
+}, 'Invalid @property rule (missing initial-value) does not overwrite previous valid rule');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`
+    @property ${name} {
+      syntax: "<color>";
+      inherits: false;
+    }
+
+    @property ${name} {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 1px;
+    }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+  });
+}, 'Previous invalid rule does not prevent valid rule from causing registration');
+
+test(() => {
+  let name = generate_name();
+
+  with_style_node(`
+    @property ${name} {
+      syntax: "<length>";
+      inherits: false;
+      initial-value: 1px;
+      quite-unknown: 200;
+    }
+  `, () => {
+    assert_equals(getComputedStyle(div).getPropertyValue(name), '1px');
+  });
+}, 'Unknown descriptors are ignored and do not invalidate rule');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/font-size-animation.html b/testing/web-platform/tests/css/css-properties-values-api/font-size-animation.html
new file mode 100644
index 0000000000..4b8ce1c255
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/font-size-animation.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/3751">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+    CSS.registerProperty({
+        name: '--length',
+        syntax: '<length>',
+        initialValue: '0px',
+        inherits: false
+    });
+</script>
+<style>
+    @keyframes font_size_animation {
+        from {
+            font-size: 10px;
+            width: 10em;
+            --length: 10em;
+        }
+        to {
+            font-size: 20px;
+            width: 20em;
+            --length: 20em;
+        }
+    }
+    #target1 {
+        font-size: 1px;
+        animation: font_size_animation 10s -5s linear paused;
+    }
+</style>
+<div id=target1></div>
+<script>
+    test(function() {
+        // At the time of writing, the correct (absolute) answer is not
+        // yet defined. However, whatever the correct answer is, there should
+        // be no difference in 'width' and a custom property registered with
+        // "<length>".
+        //
+        // See https://github.com/w3c/csswg-drafts/issues/3751
+        assert_equals(getComputedStyle(target1).getPropertyValue('width'),
+                      getComputedStyle(target1).getPropertyValue('--length'));
+    }, 'Animating font-size handled identically for standard and custom properties');
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/idlharness.html b/testing/web-platform/tests/css/css-properties-values-api/idlharness.html
new file mode 100644
index 0000000000..6f053757c3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/idlharness.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<title>CSS Properties Values API IDL tests</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/resources/WebIDLParser.js"></script>
+<script src="/resources/idlharness.js"></script>
+<script>
+  "use strict";
+
+  idl_test(
+    ["css-properties-values-api"],
+    ["cssom"]
+    // No objects
+  );
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/property-cascade.html b/testing/web-platform/tests/css/css-properties-values-api/property-cascade.html
new file mode 100644
index 0000000000..bb50213d5c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/property-cascade.html
@@ -0,0 +1,63 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#the-registerproperty-function" />
+<meta name="assert" content="Verifies that registering a property does not affect the cascade" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<style>
+
+#outer { color: rgb(1, 1, 1); }
+#inner {
+    --my-color: rgb(2, 2, 2);
+    --my-color: url(not-a-color);
+    color: var(--my-color);
+}
+
+</style>
+
+<div id=outer>
+    <div id=inner></div>
+</div>
+
+<script>
+
+test(function(){
+    // Because var(--my-color) is invalid, our color declaration should behave
+    // like color:unset, i.e. it should compute to the inherited color.
+    assert_equals(inner.computedStyleMap().get('color').toString(), 'rgb(1, 1, 1)');
+
+    CSS.registerProperty({
+        name: '--my-color',
+        syntax: '<color>',
+        initialValue: 'rgb(3, 3, 3)',
+        inherits: false
+    });
+
+    // After registering, var(--my-color) is still invalid. The important thing
+    // here is that the computed value of color is the initialValue of
+    // --my-color, and not rgb(2, 2, 2).
+    assert_equals(inner.computedStyleMap().get('color').toString(), 'rgb(3, 3, 3)');
+}, 'Registering a property does not affect cascade');
+
+test(function(){
+    CSS.registerProperty({
+        name: '--my-color-2',
+        syntax: '<color>',
+        initialValue: 'rgb(4, 4, 4)',
+        inherits: false
+    });
+
+    let element = document.createElement('div');
+    element.style = `
+        --my-color-2: rgb(2, 2, 2);
+        --my-color-2: url(not-a-color);
+        color: var(--my-color-2);
+    `;
+
+    outer.appendChild(element);
+
+    assert_equals(element.computedStyleMap().get('color').toString(), 'rgb(4, 4, 4)');
+}, 'Registering a property does not affect parsing');
+
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/register-property-syntax-parsing.html b/testing/web-platform/tests/css/css-properties-values-api/register-property-syntax-parsing.html
new file mode 100644
index 0000000000..f9022764b1
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/register-property-syntax-parsing.html
@@ -0,0 +1,222 @@
+<!DOCTYPE HTML>
+<meta charset="utf-8">
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#dom-css-registerproperty" />
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#supported-syntax-strings" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+test_count = 0;
+
+function assert_valid(syntax, initialValue) {
+    // No actual assertions, this just shouldn't throw
+    test(function() {
+        var name = '--syntax-test-' + (test_count++);
+        CSS.registerProperty({name: name, syntax: syntax, initialValue: initialValue, inherits: false});
+    }, "syntax:'" + syntax + "', initialValue:'" + initialValue + "' is valid");
+}
+
+function assert_invalid(syntax, initialValue) {
+    test(function(){
+        var name = '--syntax-test-' + (test_count++);
+        assert_throws_dom("SyntaxError",
+            () => CSS.registerProperty({name: name, syntax: syntax, initialValue: initialValue, inherits: false}));
+    }, "syntax:'" + syntax + "', initialValue:'" + initialValue + "' is invalid");
+}
+
+assert_valid("*", "a");
+assert_valid(" * ", "b");
+assert_valid("<length>", "2px");
+assert_valid(" <number>", "5");
+assert_valid("<percentage> ", "10%");
+assert_valid("<color>+", "red");
+assert_valid(" <length>+ | <percentage>", "2px 8px");
+assert_valid(" <length>+ | <color>#", "red, blue");
+assert_valid("<length>|<percentage>|<length-percentage>", "2px"); // Valid but silly
+assert_valid("<color> | <image> | <url> | <integer> | <angle>", "red");
+assert_valid("<time> | <resolution> | <transform-list> | <custom-ident>", "red");
+assert_valid("\t<color>\n|   foo", "foo");
+
+assert_valid("*", ":> hello");
+assert_valid("*", "([ brackets ]) { yay (??)}");
+assert_valid("*", "yep 'this is valid too'");
+assert_valid("*", "unmatched opening bracket is valid :(");
+assert_valid("*", '"');
+
+assert_valid("<length>", "0");
+assert_valid("<length>", "10px /*:)*/");
+assert_valid("<length>", " calc(-2px)");
+assert_valid("<length>", "calc(2px*4 + 10px)");
+assert_valid("<length>", "7.1e-4cm");
+assert_valid("<length>", "calc(7in - 12px)");
+assert_valid("<length>+", "2px 7px calc(8px)");
+assert_valid("<length>#", "2px, 7px, calc(8px)");
+assert_valid("<percentage>", "-9.3e3%");
+assert_valid("<length-percentage>", "-54%");
+assert_valid("<length-percentage>", "0");
+assert_valid("<length-percentage>", "calc(-11px + 10.4%)");
+assert_valid("<length>", "10vmin");
+assert_valid("<percentage> | <length>+", "calc(100vh - 10px) 30px");
+
+assert_valid("<number>", "-109");
+assert_valid("<number>", "2.3e4");
+assert_valid("<integer>", "-109");
+assert_valid("<integer>", "19");
+assert_valid("<integer>", "calc(1)");
+assert_valid("<integer>", "calc(1 + 2)");
+assert_valid("<integer>", "calc(3.1415)");
+assert_valid("<integer>", "calc(3.1415 + 3.1415)");
+
+assert_valid("<angle>", "10deg");
+assert_valid("<angle>", "20.5rad");
+assert_valid("<angle>", "calc(50grad + 3.14159rad)");
+assert_valid("<time>", "2s");
+assert_valid("<time>", "calc(2s - 9ms)");
+assert_valid("<resolution>", "10dpi");
+assert_valid("<resolution>", "3dPpX");
+assert_valid("<resolution>", "-5.3dpcm");
+assert_valid("<transform-function>", "translateX(2px)");
+assert_valid("<transform-function>|<integer>", "5");
+assert_valid("<transform-function>|<integer>", "scale(2)");
+assert_valid("<transform-function>+", "translateX(2px) rotate(42deg)");
+assert_valid("<transform-list>", "scale(2)");
+assert_valid("<transform-list>", "translateX(2px) rotate(20deg)");
+
+assert_valid("<color>", "rgb(12, 34, 56)");
+assert_valid("<color>", "lightgoldenrodyellow");
+assert_valid("<image>", "url(a)");
+assert_valid("<image>", "linear-gradient(yellow, blue)");
+assert_valid("<url>", "url(a)");
+
+assert_valid("banana", "banana");
+assert_valid("bAnAnA", "bAnAnA");
+assert_valid("ba-na-nya", "ba-na-nya");
+assert_valid("banana", "banan\\61");
+assert_valid("banan\\61", "banana");
+assert_valid("<custom-ident>", "banan\\61");
+assert_valid("big | bigger | BIGGER", "bigger");
+assert_valid("foo+|bar", "foo foo foo");
+
+assert_valid("banana\t", "banana");
+assert_valid("\nbanana\r\n", "banana");
+assert_valid("ba\f\n|\tna\r|nya", "nya");
+
+assert_valid(null, "null");
+assert_valid(undefined, "undefined");
+assert_valid(["array"], "array");
+
+assert_valid("\\1F914", "🤔");
+assert_valid("hmm\\1F914", "hmm🤔");
+assert_valid("\\1F914hmm", "🤔hmm");
+assert_valid("\\1F914 hmm", "🤔hmm");
+assert_valid("\\1F914\\1F914", "🤔🤔");
+
+// Invalid syntax
+assert_invalid("banana,nya", "banana");
+assert_invalid("<\\6c ength>", "10px");
+assert_invalid("<banana>", "banana");
+assert_invalid("<Number>", "10");
+assert_invalid("<length", "10px");
+assert_invalid("<LENGTH>", "10px");
+assert_invalid("< length>", "10px");
+assert_invalid("<length >", "10px");
+assert_invalid("<length> +", "10px");
+assert_invalid("<transform-list>+", "scale(2)");
+assert_invalid("<transform-list>#", "scale(2)");
+
+assert_invalid("<length>++", "10px");
+assert_invalid("<length>##", "10px");
+assert_invalid("<length>+#", "10px");
+assert_invalid("<length>#+", "10px");
+assert_invalid("<length> | *", "10px");
+assert_invalid("*|banana", "banana");
+assert_invalid("|banana", "banana");
+assert_invalid("*+", "banana");
+assert_invalid("|", "banana");
+assert_invalid(" |", "banana");
+assert_invalid("||", "banana");
+assert_invalid("foo bar", "foo bar");
+assert_invalid("foo foo foo", "foo foo foo");
+assert_invalid("foo § bar", "foo § bar");
+assert_invalid("foo \\1F914 bar", "foo \\1F914 bar");
+assert_invalid("<length> <number>", "0px 0");
+assert_invalid("<length> <length> <length>", "0px 0px 0px");
+
+assert_invalid("initial", "initial");
+assert_invalid("inherit", "inherit");
+assert_invalid("unset", "unset");
+assert_invalid("revert", "revert");
+assert_invalid("revert-layer", "revert-layer");
+assert_invalid("default", "default");
+assert_invalid("<length>|initial", "10px");
+assert_invalid("<length>|INHERIT", "10px");
+assert_invalid("<percentage>|unsEt", "2%");
+assert_invalid("<color>|REVert", "red");
+assert_invalid("<integer>|deFAUlt", "1");
+
+// Invalid initialValue
+// The 6 tests that follow are not clearly backed by the specification,
+// but (at least other than the one for 'default') they're probably a
+// good idea and we should change the spec.  See
+// https://github.com/w3c/css-houdini-drafts/issues/1076 .
+assert_invalid("*", "initial");
+assert_invalid("*", "inherit");
+assert_invalid("*", "unset");
+assert_invalid("*", "revert");
+assert_invalid("*", "revert-layer");
+assert_invalid("*", "default");
+// ... end possibly-invalid tests.
+assert_invalid("<custom-ident>", "initial");
+assert_invalid("<custom-ident>", "inherit");
+assert_invalid("<custom-ident>", "unset");
+assert_invalid("<custom-ident>", "revert");
+assert_invalid("<custom-ident>", "revert-layer");
+assert_invalid("<custom-ident>", "default");
+assert_invalid("<custom-ident>+", "foo initial bar");
+assert_invalid("<custom-ident>+", "foo inherit bar");
+assert_invalid("<custom-ident>+", "foo unset bar");
+assert_invalid("<custom-ident>+", "foo revert bar");
+assert_invalid("<custom-ident>+", "foo revert-layer bar");
+assert_invalid("<custom-ident>+", "foo default bar");
+
+assert_invalid("*", ")");
+assert_invalid("*", "([)]");
+assert_invalid("*", "whee!");
+assert_invalid("*", '"\n');
+assert_invalid("*", "url(moo '')");
+assert_invalid("*", "semi;colon");
+assert_invalid("*", "var(invalid var ref)");
+assert_invalid("*", "var(--foo)");
+
+assert_invalid("banana", "bAnAnA");
+assert_invalid("<length>", "var(--moo)");
+assert_invalid("<length>", "10");
+assert_invalid("<length>", "10%");
+assert_invalid("<length>", "calc(5px + 10%)");
+assert_invalid("<length>", "calc(5px * 3px / 6px)");
+assert_invalid("<length>", "10em");
+assert_invalid("<length>", "calc(4px + 3em)");
+assert_invalid("<length>", "calc(4px + calc(8 * 2em))");
+assert_invalid("<length>+", "calc(2ex + 16px)");
+assert_invalid("<length>+", "10px calc(20px + 4rem)");
+assert_invalid("<length>+", "");
+assert_invalid("<length>#", "");
+assert_invalid("<length>", "10px;");
+assert_invalid("<length-percentage>", "calc(2px + 10% + 7ex)");
+assert_invalid("<percentage>", "0");
+assert_invalid("<integer>", "1.0");
+assert_invalid("<integer>", "1e0");
+assert_invalid("<number>|foo", "foo var(--foo, bla)");
+assert_invalid("Foo | bar", "foo");
+assert_invalid("Foo | bar", "Bar");
+
+assert_invalid("<angle>", "0");
+assert_invalid("<angle>", "10%");
+assert_invalid("<time>", "2px");
+assert_invalid("<resolution>", "10");
+assert_invalid("<transform-function>", "scale()");
+assert_invalid("<transform-list>", "scale()");
+assert_invalid("<transform-list>+", "translateX(2px) rotate(20deg)");
+assert_invalid("<color>", "fancy-looking");
+assert_invalid("<image>", "banana.png");
+assert_invalid("<url>", "banana.png");
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/register-property.html b/testing/web-platform/tests/css/css-properties-values-api/register-property.html
new file mode 100644
index 0000000000..feb6c28d47
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/register-property.html
@@ -0,0 +1,75 @@
+<!DOCTYPE HTML>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#register-a-custom-property" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id=target></div>
+<script>
+// Tests for error checking during property registration
+
+test(function() {
+    assert_throws_js(TypeError, () => CSS.registerProperty());
+    assert_throws_js(TypeError, () => CSS.registerProperty(undefined));
+    assert_throws_js(TypeError, () => CSS.registerProperty(true));
+    assert_throws_js(TypeError, () => CSS.registerProperty(2));
+    assert_throws_js(TypeError, () => CSS.registerProperty("css"));
+    assert_throws_js(TypeError, () => CSS.registerProperty(null));
+}, "registerProperty requires a Dictionary type");
+
+test(function() {
+    // Valid property names, shouldn't throw
+    CSS.registerProperty({name: '--name1', inherits: false});
+    CSS.registerProperty({name: '--name2, no need for escapes', inherits: false});
+    CSS.registerProperty({name: ['--name', 3], inherits: false});
+
+    // Invalid property names
+    assert_throws_js(TypeError, () => CSS.registerProperty({}));
+    assert_throws_dom("SyntaxError", () => CSS.registerProperty({name: 'no-leading-dash', inherits: false}));
+    assert_throws_dom("SyntaxError", () => CSS.registerProperty({name: '', inherits: false}));
+    assert_throws_dom("SyntaxError", () => CSS.registerProperty({name: '\\--name', inherits: false}));
+}, "registerProperty requires a name matching <custom-property-name>");
+
+test(function() {
+    CSS.registerProperty({name: '--syntax-test-1', syntax: '*', inherits: false});
+    CSS.registerProperty({name: '--syntax-test-2', syntax: ' * ', inherits: false});
+    assert_throws_dom("SyntaxError",
+        () => CSS.registerProperty({name: '--syntax-test-3', syntax: 'length', inherits: false}));
+}, "registerProperty only allows omitting initialValue if syntax is '*'");
+
+test(function() {
+    CSS.registerProperty({name: '--re-register', syntax: '<length>', initialValue: '0px', inherits: false});
+    assert_throws_dom('InvalidModificationError',
+        () => CSS.registerProperty({name: '--re-register', syntax: '<percentage>', initialValue: '0%', inherits: false}));
+}, "registerProperty fails for an already registered property");
+
+test(function(){
+    CSS.registerProperty({name: '--inherit-test-1', syntax: '<length>', initialValue: '0px', inherits: true});
+    CSS.registerProperty({name: '--inherit-test-2', syntax: '<length>', initialValue: '0px', inherits: false});
+    assert_throws_js(TypeError, () => CSS.registerProperty({name: '--inherit-test-3', syntax: '<length>', initialValue: '0px'}));
+}, "registerProperty requires inherits");
+
+test(function(){
+    try {
+        let name = generate_name();
+
+        target.style.setProperty(name, 'green');
+        target.style.transitionProperty = name;
+        target.style.transitionDuration = '1s';
+        target.style.transitionTimingFunction = 'steps(1, end)';
+
+        assert_equals(getComputedStyle(target).getPropertyValue(name), 'green');
+
+        CSS.registerProperty({
+            name: name,
+            syntax: '<color>',
+            initialValue: 'red',
+            inherits: false
+        });
+
+        assert_equals(getComputedStyle(target).getPropertyValue(name), 'rgb(0, 128, 0)');
+    } finally {
+        target.style = '';
+    }
+}, 'Registering a property should not cause a transition');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/registered-properties-inheritance.html b/testing/web-platform/tests/css/css-properties-values-api/registered-properties-inheritance.html
new file mode 100644
index 0000000000..614a72a797
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/registered-properties-inheritance.html
@@ -0,0 +1,89 @@
+<!DOCTYPE HTML>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#dom-propertydescriptor-inherits" />
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#register-a-custom-property" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+#outer {
+    --inherited-length-1: 10px;
+    --inherited-length-2: var(--non-inherited-length-1);
+    --inherited-length-3: 30px;
+    --non-inherited-length-1: 22px;
+    --non-inherited-length-3: calc(var(--non-inherited-length-2) * 10);
+}
+
+#inner {
+    --inherited-length-3: 15px;
+    --non-inherited-length-1: 40px;
+    --non-inherited-length-2: 90px;
+}
+</style>
+<div id=outer><div id=inner></div></div>
+<script>
+test(function() {
+    CSS.registerProperty({name: '--inherited-length-1', syntax: '<length>', initialValue: '1px', inherits: true});
+    CSS.registerProperty({name: '--inherited-length-2', syntax: '<length>', initialValue: '2px', inherits: true});
+    CSS.registerProperty({name: '--inherited-length-3', syntax: '<length>', initialValue: '3px', inherits: true});
+    CSS.registerProperty({name: '--non-inherited-length-1', syntax: '<length>', initialValue: '4px', inherits: false});
+    CSS.registerProperty({name: '--non-inherited-length-2', syntax: '<length>', initialValue: '5px', inherits: false});
+    CSS.registerProperty({name: '--non-inherited-length-3', syntax: '<length>', initialValue: '6px', inherits: false});
+
+    outerComputedStyle = getComputedStyle(outer);
+    assert_equals(outerComputedStyle.getPropertyValue('--inherited-length-1'), '10px');
+    assert_equals(outerComputedStyle.getPropertyValue('--inherited-length-2'), '22px');
+    assert_equals(outerComputedStyle.getPropertyValue('--inherited-length-3'), '30px');
+    assert_equals(outerComputedStyle.getPropertyValue('--non-inherited-length-1'), '22px');
+    assert_equals(outerComputedStyle.getPropertyValue('--non-inherited-length-2'), '5px');
+    assert_equals(outerComputedStyle.getPropertyValue('--non-inherited-length-3'), '50px');
+
+    innerComputedStyle = getComputedStyle(inner);
+    assert_equals(innerComputedStyle.getPropertyValue('--inherited-length-1'), '10px');
+    assert_equals(innerComputedStyle.getPropertyValue('--inherited-length-2'), '22px');
+    assert_equals(innerComputedStyle.getPropertyValue('--inherited-length-3'), '15px');
+    assert_equals(innerComputedStyle.getPropertyValue('--non-inherited-length-1'), '40px');
+    assert_equals(innerComputedStyle.getPropertyValue('--non-inherited-length-2'), '90px');
+    assert_equals(innerComputedStyle.getPropertyValue('--non-inherited-length-3'), '6px');
+}, "Registered properties are correctly inherited (or not) depending on the inherits flag.");
+
+test(function(){
+    CSS.registerProperty({name: '--initial-length-1', syntax: '<length>', initialValue: '0px', inherits: false});
+    outer.style = '--initial-length-1: notalength';
+    inner.style = '--initial-length-1: inherit';
+    assert_equals(getComputedStyle(inner).getPropertyValue('--initial-length-1'), '0px');
+}, "Explicitly inheriting from a parent with an invalid value results in initial value.");
+
+test(function(){
+    CSS.registerProperty({name: '--initial-length-2', syntax: '<length>', initialValue: '0px', inherits: false});
+    inner.style = '--initial-length-2: inherit';
+    assert_equals(getComputedStyle(inner).getPropertyValue('--initial-length-2'), '0px');
+}, "Explicitly inheriting from a parent with no value results in initial value.");
+
+test(function(){
+    CSS.registerProperty({name: '--inherited-length-4', syntax: '<length>', initialValue: '0px', inherits: true});
+    outer.style = '--inherited-length-4: 42px';
+    inner.style = '--inherited-length-4: var(--undefined)';
+    assert_equals(getComputedStyle(inner).getPropertyValue('--inherited-length-4'), '42px');
+}, "Reference to undefined variable results in inherited value");
+
+test(function(){
+    CSS.registerProperty({name: '--inherited-length-5', syntax: '<length>', initialValue: '0px', inherits: true});
+    outer.style = '--inherited-length-5: 42px';
+    inner.style = '--incompatible: nolength; --inherited-length-5: var(--incompatible)';
+    assert_equals(getComputedStyle(inner).getPropertyValue('--inherited-length-5'), '42px');
+}, "Reference to syntax-incompatible variable results in inherited value");
+
+test(function(){
+    CSS.registerProperty({name: '--inherited-em', syntax: '<length>', initialValue: '0px', inherits: true});
+    outer.style = 'font-size: 11px; --inherited-em: 10em;';
+    inner.style = 'font-size: 22px; --unregistered:var(--inherited-em);';
+    assert_equals(getComputedStyle(inner).getPropertyValue('--unregistered'), '110px');
+}, "Font-relative units are absolutized before before inheritance");
+
+test(function(){
+    CSS.registerProperty({name: '--calc-length', syntax: '<length>', initialValue: '0px', inherits: true});
+    outer.style = '--calc-length: calc(10px + 10px);';
+    inner.style = '--unregistered:var(--calc-length);';
+    assert_equals(getComputedStyle(inner).getPropertyValue('--unregistered'), '20px');
+}, "Calc expressions are resolved before inheritance");
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/registered-property-change-style-001.html b/testing/web-platform/tests/css/css-properties-values-api/registered-property-change-style-001.html
new file mode 100644
index 0000000000..320b44d82e
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/registered-property-change-style-001.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS Variables Test: Style changes on registered properties using variables</title>
+<link rel="author" title="Javier Fernandez Garcia-Boente" href="mailto:jfernandez@igalia.com">
+<link rel="help" href="http://www.w3.org/TR/css-variables-1/#using-variables">
+<meta name="assert" content="A change in the custom property declaration must be propagated to all the descendants">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="outer">
+    <div id="inbetween">
+        <div id="inner"></div>
+    </div>
+</div>
+<script>
+    "use strict";
+    test( function () {
+        outer.style.cssText = '';
+        inbetween.style.cssText = '';
+        inner.style.cssText = 'color: var(--color1)';
+        let initialValue = getComputedStyle(inner).getPropertyValue('color');
+        assert_equals(initialValue, "rgb(0, 0, 0)", "Initial value");
+
+        inbetween.style.cssText = 'color: green';
+        let inheritedValue = getComputedStyle(inner).getPropertyValue('color');
+        assert_equals(inheritedValue, "rgb(0, 128, 0)", "Inherited value");
+
+        CSS.registerProperty({name: '--color1', syntax: '<color>', initialValue: 'red', inherits: true});
+        let actualValue = getComputedStyle(inner).getPropertyValue('color');
+        assert_equals(actualValue, "rgb(255, 0, 0)", "Resolved value");
+    }, "New registered property declaration");
+
+    test( function () {
+        outer.style.cssText = '';
+        inbetween.style.cssText = '';
+        inner.style.cssText = 'color: var(--color2)';
+        let initialValue = getComputedStyle(inner).getPropertyValue('color');
+        assert_equals(initialValue, "rgb(0, 0, 0)", "Initial value");
+
+        outer.style.cssText = '--color2: blue';
+        inbetween.style.cssText = 'color: green';
+        let resolvedValue = getComputedStyle(inner).getPropertyValue('color');
+        assert_equals(resolvedValue, "rgb(0, 0, 255)", "Resolved value");
+
+        outer.style.cssText = '';
+        CSS.registerProperty({name: '--color2', syntax: '<color>', initialValue: 'red', inherits: true});
+        let actualValue = getComputedStyle(inner).getPropertyValue('color');
+        assert_equals(actualValue, "rgb(255, 0, 0)", "Resolved value");
+    }, "Registered property overrides a previous declaration ");
+</script>
+
diff --git a/testing/web-platform/tests/css/css-properties-values-api/registered-property-computation.html b/testing/web-platform/tests/css/css-properties-values-api/registered-property-computation.html
new file mode 100644
index 0000000000..ec2d35c7fb
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/registered-property-computation.html
@@ -0,0 +1,175 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#calculation-of-computed-values" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+
+<style>
+#divWithFontSizeSet, #parentDiv {
+    font-size: 10px;
+    line-height: 20px;
+}
+</style>
+
+<div id=divWithFontSizeSet></div>
+<div id=parentDiv>
+    <div id=divWithFontSizeInherited></div>
+</div>
+<div id="ref"></div>
+
+<script>
+
+// Generate a property and temporarily set its value. Then call 'fn' with
+// the name of the generated property.
+function with_custom_property(element, reg, value, fn) {
+    if (element.id.length == 0)
+        throw 'The specified element must have an ID';
+
+    let name = generate_property(reg);
+
+    // Because we want to include the parsing step, insert a stylesheet
+    // node with textContent.
+    let node = document.createElement('style');
+    node.textContent = `#${element.id} { ${name}:${value}; }`;
+    document.body.append(node);
+
+    try {
+        fn(name);
+    } finally {
+        node.remove();
+    }
+}
+
+function assert_computed_value(element, syntax, value, expected) {
+    with_custom_property(element, syntax, value, (name) => {
+        let actual = getComputedStyle(element).getPropertyValue(name);
+        assert_equals(actual, expected);
+    });
+}
+
+// Computes an absolute reference value for some length.
+//
+// E.g. to figure out how many pixels '10vh' is, do length_ref('10vh').
+function length_ref(value, refnode = ref) {
+    try {
+        // The reference property 'min-height' is chosen arbitrarily, but
+        // avoid properties with "resolved value is used value"-behavior
+        // [1], as it may affect rounding, and custom properties do not
+        // have this behavior.
+        //
+        // [1] https://drafts.csswg.org/cssom/#resolved-values
+        const ref_property = 'min-height';
+        refnode.style = `${ref_property}: ${value}`;
+        return getComputedStyle(refnode).getPropertyValue(ref_property);
+    } finally {
+        refnode.style = '';
+    }
+}
+
+function test_computed_value(syntax, value, expected) {
+    test(function() {
+        assert_computed_value(divWithFontSizeSet, syntax, value, expected);
+    }, `${syntax} values are computed correctly [${value}]`);
+}
+
+test(function(){
+    const element = divWithFontSizeSet;
+    with_custom_property(element, '<length>', '14em', (name) => {
+        assert_computed_value(element, '<length>', `var(${name})`, '140px');
+    });
+}, '<length> values computed are correctly via var()-reference');
+
+test(function(){
+    const element = divWithFontSizeInherited;
+    with_custom_property(element, '<length>', '14em', (name) => {
+        assert_computed_value(element, '<length>', `var(${name})`, '140px');
+    });
+}, '<length> values computed are correctly via var()-reference when font-size is inherited');
+
+test(function(){
+    const element = divWithFontSizeInherited;
+    assert_computed_value(element, '<length>', '14em', '140px');
+}, '<length> values are computed correctly when font-size is inherited [14em]');
+
+test(function(){
+    const element = divWithFontSizeInherited;
+    assert_computed_value(element, '<length>', 'calc(14em + 10px)', '150px');
+}, '<length> values are computed correctly when font-size is inherited [calc(14em + 10px)]');
+
+test_computed_value('<length>', '12px', '12px');
+test_computed_value('<length>', '13vw', length_ref('13vw'));
+test_computed_value('<length>', '14em', '140px');
+test_computed_value('<length>', '15vmin', length_ref('15vmin'));
+test_computed_value('<length>', 'calc(16px - 7em + 10vh)', length_ref('calc(10vh - 54px)'));
+
+test_computed_value('<length>', '1in', '96px');
+test_computed_value('<length>', '2.54cm', '96px');
+test_computed_value('<length>', '25.4mm', '96px');
+test_computed_value('<length>', '6pc', '96px');
+test_computed_value('<length>', '72pt', '96px');
+
+test_computed_value('<length>', '10lh', '200px');
+
+test_computed_value('<length-percentage>', '17em', '170px');
+test_computed_value('<length-percentage>', '18%', '18%');
+test_computed_value('<length-percentage>', 'calc(19em - 2%)', 'calc(-2% + 190px)');
+
+test_computed_value('<length>#', '10px, 3em', '10px, 30px');
+test_computed_value('<length>#', '4em ,9px', '40px, 9px');
+test_computed_value('<length>#', '8em', '80px');
+
+test_computed_value('<length-percentage>#', '3% , 10vmax  , 22px', ['3%', length_ref('10vmax'), '22px'].join(', '));
+test_computed_value('<length-percentage>#', 'calc(50% + 1em), 4px', 'calc(50% + 10px), 4px');
+test_computed_value('<length-percentage>#', 'calc(13% + 37px)', 'calc(13% + 37px)');
+
+test_computed_value('<length>+', '10px 3em', '10px 30px');
+test_computed_value('<length>+', '4em 9px', '40px 9px');
+
+test_computed_value('<length-percentage>+', '3% 10vmax 22px', ['3%', length_ref('10vmax'), '22px'].join(' '));
+test_computed_value('<length-percentage>+', 'calc(50% + 1em) 4px', 'calc(50% + 10px) 4px');
+
+test_computed_value('<transform-function>', 'translateX(2px)', 'translateX(2px)');
+test_computed_value('<transform-function>', 'translateX(10em)', 'translateX(100px)');
+test_computed_value('<transform-function>', 'translateX(calc(11em + 10%))', 'translateX(calc(10% + 110px))');
+test_computed_value('<transform-function>+', 'translateX(10%) scale(2)', 'translateX(10%) scale(2)');
+
+test_computed_value('<integer>', '15', '15');
+test_computed_value('<integer>', 'calc(15 + 15)', '30');
+test_computed_value('<integer>', 'calc(2.4)', '2');
+test_computed_value('<integer>', 'calc(2.6)', '3');
+test_computed_value('<integer>', 'calc(2.6 + 3.1)', '6');
+
+test_computed_value('<integer>+', '15 calc(2.4) calc(2.6)', '15 2 3');
+
+test_computed_value('<color>', '#ff0000', 'rgb(255, 0, 0)');
+test_computed_value('<color>', '#000f00', 'rgb(0, 15, 0)');
+test_computed_value('<color>', '#00000a', 'rgb(0, 0, 10)');
+test_computed_value('<color>', '#badbee', 'rgb(186, 219, 238)');
+test_computed_value('<color>', '#badbee33', 'rgba(186, 219, 238, 0.2)');
+test_computed_value('<color>', 'tomato', 'rgb(255, 99, 71)');
+test_computed_value('<color>', 'plum', 'rgb(221, 160, 221)');
+test_computed_value('<color>', 'currentcolor', 'currentcolor');
+
+// Custom ident values that look like color keywords should not be converted.
+test_computed_value('*', 'tomato', 'tomato');
+test_computed_value('tomato | plum', 'plum', 'plum');
+test_computed_value('tomato | plum | <color>', 'plum', 'plum');
+
+test_computed_value('*', '-50grad', '-50grad');
+test_computed_value('<angle>', '180deg', '180deg');
+test_computed_value('<angle>', '400grad', '360deg');
+test_computed_value('<angle>', 'calc(360deg + 400grad)', '720deg');
+
+test_computed_value('*', '50s', '50s');
+test_computed_value('<time>', '1s', '1s');
+test_computed_value('<time>', '1000ms', '1s');
+test_computed_value('<time>', 'calc(1000ms + 1s)', '2s');
+
+test_computed_value('*', '50dpi', '50dpi');
+test_computed_value('<resolution>', '1dppx', '1dppx');
+test_computed_value('<resolution>', '96dpi', '1dppx');
+test_computed_value('<resolution>', 'calc(1dppx + 96dpi)', '2dppx');
+
+test_computed_value('*', 'url(why)', 'url(why)');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/registered-property-crosstalk.html b/testing/web-platform/tests/css/css-properties-values-api/registered-property-crosstalk.html
new file mode 100644
index 0000000000..8108894369
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/registered-property-crosstalk.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1" />
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1238686" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+
+  @property --x {
+    syntax: "<number>";
+    inherits: true;
+    initial-value: 0;
+  }
+
+  #a {
+    --y: 0;
+  }
+
+  #b {
+    --z: 0;
+  }
+
+  #c {
+    --x: 42;
+  }
+
+</style>
+
+<div id=a>
+  <div id=b>
+    <div id=c>
+    </div>
+  </div>
+</div>
+
+<script>
+
+test(function(){
+  assert_equals(getComputedStyle(a).getPropertyValue('--x'), '0');
+  assert_equals(getComputedStyle(b).getPropertyValue('--x'), '0');
+  assert_equals(getComputedStyle(c).getPropertyValue('--x'), '42');
+}, 'Only #c should be affected by --x:42');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/registered-property-cssom.html b/testing/web-platform/tests/css/css-properties-values-api/registered-property-cssom.html
new file mode 100644
index 0000000000..6231e27e77
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/registered-property-cssom.html
@@ -0,0 +1,91 @@
+<!DOCTYPE HTML>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#dom-css-registerproperty" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<style>
+#inner {
+  --length: 10px;
+  --color: red;
+}
+#outer {
+  --length: 77px;
+  --color: blue;
+}
+</style>
+
+<div id=outer>
+  <div id=inner></div>
+</div>
+
+<script>
+var computedStyle = getComputedStyle(inner);
+var inlineStyle = inner.style;
+var sheetStyle = document.styleSheets[0].cssRules[0].style;
+
+test(function() {
+  // Nothing registered yet, whatever you specify works
+  assert_equals(computedStyle.getPropertyValue('--length'), '10px');
+  assert_equals(computedStyle.getPropertyValue('--color'), 'red');
+}, "Initially unregistered values from stylesheet");
+
+test(function() {
+  inlineStyle.setProperty('--length', '5');
+  inlineStyle.setProperty('--color', 'hello');
+
+  assert_equals(inlineStyle.getPropertyValue('--length'), '5');
+  assert_equals(inlineStyle.getPropertyValue('--color'), 'hello');
+  assert_equals(computedStyle.getPropertyValue('--length'), '5');
+  assert_equals(computedStyle.getPropertyValue('--color'), 'hello');
+}, "CSSOM setters function as expected for unregistered properties");
+
+test(function() {
+  CSS.registerProperty({name: '--length', syntax: '<length>', initialValue: '0px', inherits: false});
+  CSS.registerProperty({name: '--color', syntax: '<color>', initialValue: 'white', inherits: true});
+}, "CSS.registerProperty");
+
+test(function() {
+  assert_equals(inlineStyle.getPropertyValue('--length'), '5');
+  assert_equals(inlineStyle.getPropertyValue('--color'), 'hello');
+  assert_equals(computedStyle.getPropertyValue('--length'), '0px');
+  assert_equals(computedStyle.getPropertyValue('--color'), 'rgb(0, 0, 255)');
+}, "Formerly valid values are still readable from inline styles but are computed as the unset value");
+
+test(function() {
+  inlineStyle.setProperty('--length', 'hi');
+  inlineStyle.setProperty('--color', '20');
+  assert_equals(inlineStyle.getPropertyValue('--length'), 'hi');
+  assert_equals(inlineStyle.getPropertyValue('--color'), '20');
+}, "Values not matching the registered type can still be set");
+
+test(function() {
+  inlineStyle.removeProperty('--length');
+  inlineStyle.setProperty('--color', '');
+  assert_equals(inlineStyle.getPropertyValue('--length'), '');
+  assert_equals(inlineStyle.getPropertyValue('--color'), '');
+  assert_equals(computedStyle.getPropertyValue('--length'), '10px');
+  assert_equals(computedStyle.getPropertyValue('--color'), 'rgb(255, 0, 0)');
+}, "Values can be removed from inline styles");
+
+test(function() {
+  // 'banana' is not a valid <length>, but still accepted.
+  sheetStyle.setProperty('--length', 'banana');
+  assert_equals(computedStyle.getPropertyValue('--length'), '0px');
+  sheetStyle.setProperty('--length', '20px');
+  assert_equals(computedStyle.getPropertyValue('--length'), '20px');
+  sheetStyle.setProperty('--length', 'initial');
+  assert_equals(computedStyle.getPropertyValue('--length'), '0px');
+}, "Stylesheets can be modified by CSSOM");
+
+test(function() {
+  inlineStyle.setProperty('--length', '30px');
+  inlineStyle.setProperty('--color', 'pink');
+  assert_equals(inlineStyle.getPropertyValue('--length'), '30px');
+  assert_equals(inlineStyle.getPropertyValue('--color'), 'pink');
+  assert_equals(computedStyle.getPropertyValue('--length'), '30px');
+  assert_equals(computedStyle.getPropertyValue('--color'), 'rgb(255, 192, 203)');
+  inlineStyle.setProperty('--color', 'inherit');
+  assert_equals(inlineStyle.getPropertyValue('--color'), 'inherit');
+  assert_equals(computedStyle.getPropertyValue('--color'), 'rgb(0, 0, 255)');
+}, "Valid values can be set on inline styles");
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/registered-property-initial.html b/testing/web-platform/tests/css/css-properties-values-api/registered-property-initial.html
new file mode 100644
index 0000000000..2fa062f310
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/registered-property-initial.html
@@ -0,0 +1,71 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#dom-propertydescriptor-initialvalue" />
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#register-a-custom-property" />
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#substitution" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id=target></div>
+<script>
+
+function test_initial_value(reg, expected) {
+    let suffix = reg.inherits === true ? ', inherits' : '';
+    test(function(){
+        let name = generate_property(reg);
+        let actual = getComputedStyle(target).getPropertyValue(name);
+        assert_equals(actual, expected);
+    }, `Initial value for ${reg.syntax} correctly computed [${reg.initialValue}${suffix}]`);
+}
+
+test_initial_value({ syntax: '<length>', initialValue: 'calc(10px + 15px)' }, '25px');
+test_initial_value({ syntax: '<length>', initialValue: '1in' }, '96px');
+test_initial_value({ syntax: '<length>', initialValue: '2.54cm' }, '96px');
+test_initial_value({ syntax: '<length>', initialValue: '25.4mm' }, '96px');
+test_initial_value({ syntax: '<length>', initialValue: '6pc' }, '96px');
+test_initial_value({ syntax: '<length>', initialValue: '72pt' }, '96px');
+test_initial_value({ syntax: '<percentage>', initialValue: 'calc(10% + 20%)' }, '30%');
+test_initial_value({ syntax: '<length-percentage>', initialValue: 'calc(1in + 10% + 4px)' }, 'calc(10% + 100px)');
+test_initial_value({ syntax: '<color>', initialValue: 'pink', inherits: true }, 'rgb(255, 192, 203)');
+test_initial_value({ syntax: '<color>', initialValue: 'purple' }, 'rgb(128, 0, 128)');
+test_initial_value({ syntax: '<transform-function>', initialValue: 'rotate(42deg)' }, 'rotate(42deg)');
+test_initial_value({ syntax: '<transform-list>', initialValue: 'scale(calc(2 + 2))' }, 'scale(4)');
+test_initial_value({ syntax: '<transform-list>', initialValue: 'scale(calc(2 + 1)) translateX(calc(3px + 1px))' }, 'scale(3) translateX(4px)');
+test_initial_value({ syntax: '<url>', initialValue: 'url(a)' },
+    `url("${new URL('a', document.baseURI)}")`);
+test_initial_value({ syntax: '<url>+', initialValue: 'url(a) url(a)' },
+    `url("${new URL('a', document.baseURI)}") url("${new URL('a', document.baseURI)}")`);
+
+// Test that the initial value of the custom property 'reg' is successfully
+// substituted into 'property'.
+function test_substituted_value(reg, property, expected) {
+    let inherits_text = reg.inherits === true ? 'inherited' : 'non-inherited';
+    test(function(){
+        try {
+            let name = generate_property(reg);
+            target.style = `${property}:var(${name});`;
+            assert_equals(getComputedStyle(target).getPropertyValue(property), expected);
+        } finally {
+            target.style = '';
+        }
+    }, `Initial ${inherits_text} value can be substituted [${reg.initialValue}, ${property}]`);
+}
+
+test_substituted_value({ syntax: '<color>', initialValue: 'purple', inherits: true }, 'color', 'rgb(128, 0, 128)');
+test_substituted_value({ syntax: '<color>', initialValue: 'pink' }, 'background-color', 'rgb(255, 192, 203)');
+
+// Registered properties shall substitute as a token sequence equivalent to
+// their computed value.
+test_substituted_value({ syntax: 'foo', initialValue: '\tfoo\t' }, '--x', 'foo');
+test_substituted_value({ syntax: '<angle>', initialValue: '\t1turn' }, '--x', '360deg');
+test_substituted_value({ syntax: '<color>', initialValue: ' pink ' }, '--x', 'rgb(255, 192, 203)');
+test_substituted_value({ syntax: '<custom-ident>', initialValue: '\ttest' }, '--x', 'test');
+test_substituted_value({ syntax: '<integer>', initialValue: 'calc(20 + 20 + 10)' }, '--x', '50');
+test_substituted_value({ syntax: '<length-percentage>', initialValue: '\tcalc(13% + 37px)' }, '--x', 'calc(13% + 37px)');
+test_substituted_value({ syntax: '<length>', initialValue: 'calc(10px + 15px)' }, '--x', '25px');
+test_substituted_value({ syntax: '<number>', initialValue: 'calc(13 + 37)' }, '--x', '50');
+test_substituted_value({ syntax: '<percentage>', initialValue: 'calc(13% + 37%)' }, '--x', '50%');
+test_substituted_value({ syntax: '<time>', initialValue: '2000ms' }, '--x', '2s');
+test_substituted_value({ syntax: '<transform-function>', initialValue: 'scale(calc(2 + 2))' }, '--x', 'scale(4)');
+test_substituted_value({ syntax: '<transform-list>', initialValue: 'scale(calc(2 + 2)) translateX(calc(3px + 1px))' }, '--x', 'scale(4) translateX(4px)');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/registered-property-revert.html b/testing/web-platform/tests/css/css-properties-values-api/registered-property-revert.html
new file mode 100644
index 0000000000..33c395d2c2
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/registered-property-revert.html
@@ -0,0 +1,94 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1" />
+<link rel="help" href="https://drafts.csswg.org/css-cascade/#default" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+    #parent {
+        --inherited: 10px;
+        --non-inherited: 10px;
+    }
+    #child {
+        --inherited: 20px;
+        --non-inherited: 20px;
+        --inherited: revert;
+        --non-inherited: revert;
+    }
+
+    @keyframes revert_animation {
+        from {
+            --animated-inherited: revert;
+            --animated-non-inherited: revert;
+        }
+        to {
+            --animated-inherited: 100px;
+            --animated-non-inherited: 100px;
+        }
+    }
+
+    #animated_parent {
+        --animated-inherited: 0px;
+    }
+    #animated_child {
+        animation: revert_animation 10s -5s linear paused;
+    }
+</style>
+<div id=parent>
+    <div id=child>
+    </div>
+</div>
+<div id=animated_parent>
+    <div id=animated_child>
+    </div>
+</div>
+<script>
+
+CSS.registerProperty({
+    name: "--inherited",
+    syntax: "<length>",
+    initialValue: "0px",
+    inherits: true
+});
+
+CSS.registerProperty({
+    name: "--non-inherited",
+    syntax: "<length>",
+    initialValue: "0px",
+    inherits: false
+});
+
+CSS.registerProperty({
+    name: "--animated-non-inherited",
+    syntax: "<length>",
+    initialValue: "0px",
+    inherits: false
+});
+
+CSS.registerProperty({
+    name: "--animated-inherited",
+    syntax: "<length>",
+    initialValue: "10000px",
+    inherits: true
+});
+
+test(function(){
+    let cs = getComputedStyle(child);
+    assert_equals(cs.getPropertyValue('--inherited'), '10px');
+}, 'Inherited registered custom property can be reverted');
+
+test(function(){
+    let cs = getComputedStyle(child);
+    assert_equals(cs.getPropertyValue('--non-inherited'), '0px');
+}, 'Non-inherited registered custom property can be reverted');
+
+test(function(){
+    let cs = getComputedStyle(animated_child);
+    assert_equals(cs.getPropertyValue('--animated-non-inherited'), '50px');
+}, 'Non-inherited registered custom property can be reverted in animation');
+
+test(function(){
+    let cs = getComputedStyle(animated_child);
+    assert_equals(cs.getPropertyValue('--animated-inherited'), '50px');
+}, 'Inherited registered custom property can be reverted in animation');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/resources/utils.js b/testing/web-platform/tests/css/css-properties-values-api/resources/utils.js
new file mode 100644
index 0000000000..dbbda1d585
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/resources/utils.js
@@ -0,0 +1,237 @@
+let next_property_id = 1;
+
+// Generate a unique property name on the form --prop-N.
+function generate_name() {
+  return `--prop-${next_property_id++}`;
+}
+
+// Produce a compatible initial value for the specified syntax.
+function any_initial_value(syntax) {
+  let components = syntax.split('|').map(x => x.trim())
+  let first_component = components[0];
+
+  if (first_component.endsWith('+') || first_component.endsWith('#'))
+    first_component = first_component.slice(0, -1);
+
+  switch (first_component) {
+    case '*':
+    case '<custom-ident>':
+      return 'NULL';
+    case '<angle>':
+      return '0deg';
+    case '<color>':
+      return 'rgb(0, 0, 0)';
+    case '<image>':
+    case '<url>':
+      return 'url(0)';
+    case '<integer>':
+    case '<length-percentage>':
+    case '<length>':
+    case '<number>':
+      return '0';
+    case '<percentage>':
+      return '0%';
+    case '<resolution>':
+      return '0dpi';
+    case '<time>':
+      return '0s';
+    case '<transform-function>':
+    case '<transform-list>':
+      return 'matrix(0, 0, 0, 0, 0, 0)';
+    default:
+      // We assume syntax is a specific custom ident.
+      return first_component;
+  }
+}
+
+// Registers a unique property on the form '--prop-N' and returns the name.
+// Any value except 'syntax' may be omitted, in which case the property will
+// not inherit, and some undefined (but compatible) initial value will be
+// generated. If a single string is used as the argument, it is assumed to be
+// the syntax.
+function generate_property(reg) {
+  // Verify that only valid keys are specified. This prevents the caller from
+  // accidentally supplying 'inherited' instead of 'inherits', for example.
+  if (typeof(reg) === 'object') {
+    const permitted = new Set(['name', 'syntax', 'initialValue', 'inherits']);
+    if (!Object.keys(reg).every(k => permitted.has(k)))
+      throw new Error('generate_property: invalid parameter');
+  }
+
+  let syntax = typeof(reg) === 'string' ? reg : reg.syntax;
+  let initial = typeof(reg.initialValue) === 'undefined' ? any_initial_value(syntax)
+                                                         : reg.initialValue;
+  let inherits = typeof(reg.inherits) === 'undefined' ? false : reg.inherits;
+
+  let name = generate_name();
+  CSS.registerProperty({
+    name: name,
+    syntax: syntax,
+    initialValue: initial,
+    inherits: inherits
+  });
+  return name;
+}
+
+function all_syntaxes() {
+  return [
+    '*',
+    '<angle>',
+    '<color>',
+    '<custom-ident>',
+    '<image>',
+    '<integer>',
+    '<length-percentage>',
+    '<length>',
+    '<number>',
+    '<percentage>',
+    '<resolution>',
+    '<time>',
+    '<transform-function>',
+    '<transform-list>',
+    '<url>'
+  ]
+}
+
+function with_style_node(text, fn) {
+  let node = document.createElement('style');
+  node.textContent = text;
+  try {
+    document.body.append(node);
+    fn(node);
+  } finally {
+    node.remove();
+  }
+}
+
+function with_at_property(desc, fn) {
+  let name = typeof(desc.name) === 'undefined' ? generate_name() : desc.name;
+  let text = `@property ${name} {`;
+  if (typeof(desc.syntax) !== 'undefined')
+    text += `syntax:${desc.syntax};`;
+  if (typeof(desc.initialValue) !== 'undefined')
+    text += `initial-value:${desc.initialValue};`;
+  if (typeof(desc.inherits) !== 'undefined')
+    text += `inherits:${desc.inherits};`;
+  text += '}';
+  with_style_node(text, (node) => fn(name, node.sheet.rules[0]));
+}
+
+function test_with_at_property(desc, fn, description) {
+  test(() => with_at_property(desc, fn), description);
+}
+
+function test_with_style_node(text, fn, description) {
+  test(() => with_style_node(text, fn), description);
+}
+
+function animation_test(property, values, description) {
+  const name = generate_name();
+  property.name = name;
+  CSS.registerProperty(property);
+
+  test(() => {
+    const duration = 1000;
+    const keyframes = {};
+    keyframes[name] = values.keyframes;
+
+    const iterations = 3;
+    const composite = values.composite || "replace";
+    const iterationComposite = values.iterationComposite || "replace";
+    const animation = target.animate(keyframes, { composite, iterationComposite, iterations, duration });
+    animation.pause();
+    // We seek to the middle of the third iteration which will allow to test cases where
+    // iterationComposite is set to something other than "replace".
+    animation.currentTime = duration * 2.5;
+
+    assert_equals(getComputedStyle(target).getPropertyValue(name), values.expected);
+  }, description);
+};
+
+function discrete_animation_test(syntax, fromValue, toValue, description) {
+  test(() => {
+    const name = generate_name();
+
+    CSS.registerProperty({
+      name,
+      syntax,
+      inherits: false,
+      initialValue: fromValue
+    });
+
+    const duration = 1000;
+    const keyframes = [];
+    keyframes[name] = toValue;
+    const animation = target.animate(keyframes, duration);
+    animation.pause();
+
+    const checkAtProgress = (progress, expected) => {
+      animation.currentTime = duration * 0.25;
+      assert_equals(getComputedStyle(target).getPropertyValue(name), fromValue, `The correct value is used at progress = ${progress}`);
+    };
+
+    checkAtProgress(0, fromValue);
+    checkAtProgress(0.25, fromValue);
+    checkAtProgress(0.49, fromValue);
+    checkAtProgress(0.5, toValue);
+    checkAtProgress(0.75, toValue);
+    checkAtProgress(1, toValue);
+  }, description || `Animating a custom property of type ${syntax} is discrete`);
+}
+
+function transition_test(options, description) {
+  promise_test(async () => {
+    const customProperty = generate_name();
+
+    CSS.registerProperty({
+      name: customProperty,
+      syntax: options.syntax,
+      inherits: false,
+      initialValue: options.from
+    });
+
+    assert_equals(getComputedStyle(target).getPropertyValue(customProperty), options.from, "Element has the expected initial value");
+
+    const transitionEventPromise = new Promise(resolve => {
+      target.addEventListener("transitionrun", event => {
+          assert_equals(event.propertyName, customProperty, "TransitionEvent has the expected property name");
+          resolve();
+      });
+    });
+
+    target.style.transition = `${customProperty} 1s -500ms linear`;
+    target.style.setProperty(customProperty, options.to);
+
+    const animations = target.getAnimations();
+    assert_equals(animations.length, 1, "A single animation is running");
+
+    const transition = animations[0];
+    assert_class_string(transition, "CSSTransition", "A CSSTransition is running");
+
+    transition.pause();
+    assert_equals(getComputedStyle(target).getPropertyValue(customProperty), options.expected, "Element has the expected animated value");
+
+    await transitionEventPromise;
+  }, description);
+}
+
+function no_transition_test(options, description) {
+  test(() => {
+    const customProperty = generate_name();
+
+    CSS.registerProperty({
+      name: customProperty,
+      syntax: options.syntax,
+      inherits: false,
+      initialValue: options.from
+    });
+
+    assert_equals(getComputedStyle(target).getPropertyValue(customProperty), options.from, "Element has the expected initial value");
+
+    target.style.transition = `${customProperty} 1s -500ms linear`;
+    target.style.setProperty(customProperty, options.to);
+
+    assert_equals(target.getAnimations().length, 0, "No animation was created");
+    assert_equals(getComputedStyle(target).getPropertyValue(customProperty), options.to, "Element has the expected final value");
+  }, description);
+};
diff --git a/testing/web-platform/tests/css/css-properties-values-api/self-utils.html b/testing/web-platform/tests/css/css-properties-values-api/self-utils.html
new file mode 100644
index 0000000000..b770c86c38
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/self-utils.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<title>Self-test for utils.js</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<div id=outer><div id=inner></div></div>
+<script>
+
+test(function(){
+  let syntaxes = all_syntaxes().concat([
+    'foo',
+    'bar | <length>',
+    '<angle> | <length>'
+  ]);
+  // Don't throw:
+  syntaxes.forEach(generate_property);
+}, 'Default initial values of generated properties are valid (self-test).');
+
+test(function(){
+  try {
+    let inherited = generate_property({ syntax: '<length>', inherits: true });
+    let non_inherited = generate_property({ syntax: '<length>', inherits: false, initialValue: '5px' });
+    outer.style = `${inherited}: 10px; ${non_inherited}: 11px;`;
+    assert_equals(getComputedStyle(outer).getPropertyValue(inherited), '10px');
+    assert_equals(getComputedStyle(outer).getPropertyValue(non_inherited), '11px');
+    assert_equals(getComputedStyle(inner).getPropertyValue(inherited), '10px');
+    assert_equals(getComputedStyle(inner).getPropertyValue(non_inherited), '5px');
+  } finally {
+    outer.style = '';
+    inner.style = '';
+  }
+}, 'Generated properties respect inherits flag');
+
+test(function(){
+  assert_throws_js(Error, () => generate_property({syntax: '<length>', foo: 1}));
+  assert_throws_js(Error, () => generate_property({syntax: '<length>', inherited: false}));
+  assert_throws_js(Error, () => generate_property({syntax: '<length>', initial: '10px'}));
+}, 'Can\'t generate property with unknown fields');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.css b/testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.css
new file mode 100644
index 0000000000..aeb6ad5abe
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.css
@@ -0,0 +1,4 @@
+#target {
+  --reg-alt-non-inherited-url: url(foo.jpg);
+  --reg-alt-non-inherited-func: url("foo.jpg");
+}
diff --git a/testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.js b/testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.js
new file mode 100644
index 0000000000..f25e18beeb
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/support/alt/alt.js
@@ -0,0 +1,11 @@
+function reg_url(name, inherits) {
+    CSS.registerProperty({
+        name: name,
+        syntax: '<url> | none',
+        inherits: inherits,
+        initialValue: 'none'
+    });
+}
+
+reg_url('--reg-alt-non-inherited-url', false);
+reg_url('--reg-alt-non-inherited-func', false);
diff --git a/testing/web-platform/tests/css/css-properties-values-api/support/main/main.css b/testing/web-platform/tests/css/css-properties-values-api/support/main/main.css
new file mode 100644
index 0000000000..6b81abdee7
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/support/main/main.css
@@ -0,0 +1,22 @@
+#target {
+    --unreg-url: url(foo.jpg);
+    --unreg-func: url("foo.jpg");
+
+    --reg-inherited-url: url(foo.jpg);
+    --reg-non-inherited-url: url(foo.jpg);
+
+    --reg-inherited-func: url("foo.jpg");
+    --reg-non-inherited-func: url("foo.jpg");
+
+    --reg-ref-to-unreg-url: var(--unreg-url);
+    --reg-ref-to-unreg-func: var(--unreg-func);
+
+    --reg-ref-to-reg-url: var(--reg-alt-non-inherited-url);
+    --reg-ref-to-reg-func: var(--reg-alt-non-inherited-func);
+
+    --unreg-ref-to-reg-url: var(--reg-alt-non-inherited-url);
+    --unreg-ref-to-reg-func: var(--reg-alt-non-inherited-func);
+
+    --unreg-multi-ref-to-reg-urls: var(--reg-non-inherited-url), var(--reg-alt-non-inherited-url);
+    --unreg-multi-ref-to-reg-funcs: var(--reg-non-inherited-func), var(--reg-alt-non-inherited-func);
+}
diff --git a/testing/web-platform/tests/css/css-properties-values-api/support/main/main.js b/testing/web-platform/tests/css/css-properties-values-api/support/main/main.js
new file mode 100644
index 0000000000..169ed7b53c
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/support/main/main.js
@@ -0,0 +1,25 @@
+function reg_url(name, inherits) {
+    CSS.registerProperty({
+        name: name,
+        syntax: '<url> | none',
+        inherits: inherits,
+        initialValue: 'none'
+    });
+}
+
+reg_url('--reg-non-inherited-url', false);
+reg_url('--reg-non-inherited-func', false);
+
+reg_url('--reg-inherited-url', true);
+reg_url('--reg-inherited-func', true);
+
+reg_url('--reg-ref-to-unreg-url', false);
+reg_url('--reg-ref-to-unreg-func', false);
+
+reg_url('--reg-ref-to-reg-url', false);
+reg_url('--reg-ref-to-reg-func', false);
+
+reg_url('--reg-merged-func', false);
+
+reg_url('--reg-utf16be-url', false);
+reg_url('--reg-utf16be-func', false);
diff --git a/testing/web-platform/tests/css/css-properties-values-api/support/main/main.utf16be.css b/testing/web-platform/tests/css/css-properties-values-api/support/main/main.utf16be.css
new file mode 100644
index 0000000000..26485da32b
Binary files /dev/null and b/testing/web-platform/tests/css/css-properties-values-api/support/main/main.utf16be.css differ
diff --git a/testing/web-platform/tests/css/css-properties-values-api/typedom.html b/testing/web-platform/tests/css/css-properties-values-api/typedom.html
new file mode 100644
index 0000000000..51940e298d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/typedom.html
@@ -0,0 +1,993 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#css-style-value-reification" />
+<meta name="assert" content="Verifies that registered custom properties interact correctly with CSS Typed OM" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<style id=style>
+    div {}
+</style>
+<div id=target></div>
+
+<script>
+
+// Cleans style rules used for testing between every test.
+add_result_callback(function(){
+    target.attributeStyleMap.clear();
+    // Clears 'div' rule in #style:
+    style.sheet.rules[0].styleMap.clear();
+});
+
+// In the following utility functions, the 'map' parameter (if present)
+// can be any StylePropertyMap. (Not StylePropertyMapReadOnly).
+
+// Verifies that get()/getAll() reifies the specified property to a
+// CSSUnparsedValue, with a string serialization equal to 'value'.
+function verify_map_get_unparsed(map, name, value) {
+    map.set(name, value);
+
+    let specifiedValue = map.get(name);
+    assert_true(specifiedValue instanceof CSSUnparsedValue);
+    assert_equals(specifiedValue.toString(), value);
+
+    let allSpecifiedValues = map.getAll(name);
+    assert_equals(allSpecifiedValues.length, 1);
+    assert_true(allSpecifiedValues[0] instanceof CSSUnparsedValue);
+    assert_equals(allSpecifiedValues[0].toString(), value);
+}
+
+// Verifies that the specified value is accepted by set().
+function verify_map_set(map, name, value) {
+    map.set(name, value);
+    assert_equals(map.get(name).toString(), value.toString());
+}
+
+// Verifies that the specified value is NOT accepted by set().
+function verify_map_not_set(map, name, value) {
+    assert_throws_js(TypeError, () => {
+        map.set(name, value);
+    });
+}
+
+// Verifies that the specified value is NOT accepted by append().
+function verify_map_no_append(map, name, value) {
+    assert_throws_js(TypeError, () => {
+        map.append(name, value);
+    });
+}
+
+// Verifies that the property 'name' shows up on iteration, that it's reified
+// as a CSSUnparsedValue, and that the string representation is equal to
+// 'value'.
+function verify_map_iteration_unparsed(map, name, value) {
+    map.set(name, value);
+    let result = Array.from(map).filter(e => e[0] == name)[0];
+    assert_equals(result.length, 2);
+    let iter_value = result[1];
+    assert_equals(iter_value.length, 1);
+    assert_true(iter_value[0] instanceof CSSUnparsedValue);
+    assert_equals(iter_value[0].toString(), value);
+}
+
+// Verifies that CSSStyleValue.parse/parseAll results in a CSSStyleValue with
+// the 'expected' type.
+function verify_parsed_type(prop, value, expected) {
+    let parse_value = CSSStyleValue.parse(prop, value);
+    let parse_all_value = CSSStyleValue.parseAll(prop, value);
+
+    assert_true(parse_value instanceof expected);
+    assert_true(parse_all_value.every(x => x instanceof expected))
+}
+
+// On the target element, verify that computed value of 'name' is an instance
+// of 'expected' and not an instance of CSSUnparsedValue.
+//
+// If 'value' is non-null, that value is first set using the style attribute
+// of the target element.
+function verify_computed_type(name, value, expected) {
+    if (expected == CSSUnparsedValue) {
+        throw 'CSSUnparsedValue may not be used as expected type';
+    }
+
+    try {
+        if (value != null) {
+            target.style = `${name}: ${value}`;
+        }
+
+        let computedValue = target.computedStyleMap().get(name);
+
+        assert_false(computedValue instanceof CSSUnparsedValue);
+        assert_true(computedValue instanceof expected);
+    } finally {
+        if (value != null) {
+            target.style = '';
+        }
+    }
+}
+
+// Verifies that the property 'name' shows up on iteration, that it's reified
+// to the specified type, and that the string representation is equal to 'value'.
+function verify_computed_iteration_type(name, value, type) {
+    target.attributeStyleMap.set(name, value);
+    let result = Array.from(target.computedStyleMap()).filter(e => e[0] == name)[0];
+    assert_equals(result.length, 2);
+    let iter_value = result[1];
+    assert_equals(iter_value.length, 1);
+    assert_true(iter_value[0] instanceof type);
+    assert_equals(iter_value[0].toString(), value);
+}
+
+// Run the same test twice: once for each StylePropertyMap.
+//
+// https://drafts.css-houdini.org/css-typed-om-1/#stylepropertymap
+function test_specified_maps(func, description) {
+    test(function(){
+        func(target.attributeStyleMap)
+    }, description + ' [attributeStyleMap]');
+
+    test(function(){
+        let rule = style.sheet.rules[0];
+        func(rule.styleMap)
+    }, description + ' [styleMap]');
+}
+
+// StylePropertyMapReadOnly.get
+
+test(function(){
+    let name = generate_property('*', 'if(){}');
+    assert_true(target.computedStyleMap().get(name) instanceof CSSUnparsedValue);
+
+    target.attributeStyleMap.set(name, 'as{}df');
+    assert_true(target.computedStyleMap().get(name) instanceof CSSUnparsedValue);
+    target.attributeStyleMap.delete(name);
+}, 'Computed * is reified as CSSUnparsedValue');
+
+test(function(){
+    verify_computed_type(generate_property('<angle>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <angle> '), '42deg', CSSUnitValue);
+}, 'Computed <angle> is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<color>'), null, CSSStyleValue);
+    verify_computed_type(generate_property('fail | <color> '), null, CSSStyleValue);
+}, 'Computed <color> is reified as CSSStyleValue');
+
+test(function(){
+    verify_computed_type(generate_property('<custom-ident>'), null, CSSKeywordValue);
+    verify_computed_type(generate_property('<custom-ident> | <length>'), 'none', CSSKeywordValue);
+}, 'Computed <custom-ident> is reified as CSSKeywordValue');
+
+test(function(){
+    verify_computed_type(generate_property('<image>'), null, CSSImageValue);
+    verify_computed_type(generate_property('fail | <image> '), 'url(thing.png)', CSSImageValue);
+}, 'Computed <image> [url] is reified as CSSImageValue');
+
+test(function(){
+    verify_computed_type(generate_property('<integer>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <integer> '), '100', CSSUnitValue);
+}, 'Computed <integer> is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<length-percentage>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <length-percentage> '), '10%', CSSUnitValue);
+}, 'Computed <length-percentage> [%] is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<length-percentage>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <length-percentage> '), '10px', CSSUnitValue);
+}, 'Computed <length-percentage> [px] is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property({syntax: '<length-percentage>', initialValue: 'calc(10% + 10px)'}), null, CSSMathSum);
+    verify_computed_type(generate_property('fail | <length-percentage> '), 'calc(10px + 10%)', CSSMathSum);
+}, 'Computed <length-percentage> [px + %] is reified as CSSMathSum');
+
+test(function(){
+    verify_computed_type(generate_property('<length>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <length> '), '10px', CSSUnitValue);
+}, 'Computed <length> is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<number>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <number> '), '42', CSSUnitValue);
+}, 'Computed <number> is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<percentage>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <percentage> '), '10%', CSSUnitValue);
+}, 'Computed <percentage> is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<resolution>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <resolution> '), '300dpi', CSSUnitValue);
+}, 'Computed <resolution> is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<time>'), null, CSSUnitValue);
+    verify_computed_type(generate_property('fail | <time> '), '42s', CSSUnitValue);
+}, 'Computed <time> is reified as CSSUnitValue');
+
+test(function(){
+    verify_computed_type(generate_property('<url>'), null, CSSStyleValue);
+    verify_computed_type(generate_property('fail | <url> '), 'url(a)', CSSStyleValue);
+}, 'Computed <url> is reified as CSSStyleValue');
+
+test(function(){
+    verify_computed_type(generate_property('thing1 | THING2'), null, CSSKeywordValue);
+    verify_computed_type(generate_property('thing1 | THING2 | <url>'), 'THING2', CSSKeywordValue);
+}, 'Computed ident is reified as CSSKeywordValue');
+
+test(function(){
+    verify_computed_type(generate_property('<length>+'), null, CSSUnitValue);
+    verify_computed_type(generate_property('<length>+'), '10px 20px', CSSUnitValue);
+}, 'First computed value correctly reified in space-separated list');
+
+test(function(){
+    verify_computed_type(generate_property('<length>#'), null, CSSUnitValue);
+    verify_computed_type(generate_property('<length>#'), '10px, 20px', CSSUnitValue);
+}, 'First computed value correctly reified in comma-separated list');
+
+// StylePropertyMapReadOnly.getAll
+
+test(function(){
+    let name = generate_property({syntax: '<length>+', initialValue: '10px 20px'});
+    assert_equals(target.computedStyleMap().getAll(name).length, 2);
+    assert_true(target.computedStyleMap().getAll(name).every(x => x instanceof CSSUnitValue));
+
+    target.style = `${name}: 10px 20px 30px`;
+    assert_equals(target.computedStyleMap().getAll(name).length, 3);
+    assert_true(target.computedStyleMap().getAll(name).every(x => x instanceof CSSUnitValue));
+}, 'All computed values correctly reified in space-separated list');
+
+test(function(){
+    let name = generate_property({syntax: '<length>#', initialValue: '10px, 20px'});
+    assert_equals(target.computedStyleMap().getAll(name).length, 2);
+    assert_true(target.computedStyleMap().getAll(name).every(x => x instanceof CSSUnitValue));
+
+    target.style = `${name}: 10px, 20px, 30px`;
+    assert_equals(target.computedStyleMap().getAll(name).length, 3);
+    assert_true(target.computedStyleMap().getAll(name).every(x => x instanceof CSSUnitValue));
+}, 'All computed values correctly reified in comma-separated list');
+
+// StylePropertyMap.get/All
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('*'), 'foo');
+}, 'Specified * is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('foo'), 'foo');
+}, 'Specified foo is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<angle>'), '10deg');
+}, 'Specified <angle> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<color>'), 'green');
+}, 'Specified <color> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<custom-ident>'), 'foo');
+}, 'Specified <custom-ident> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<image>'), 'url("a")');
+}, 'Specified <image> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<integer>'), '1');
+}, 'Specified <integer> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<length-percentage>'), 'calc(10% + 10px)');
+}, 'Specified <length-percentage> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<length>'), '10px');
+}, 'Specified <length> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<number>'), '1');
+}, 'Specified <number> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<percentage>'), '10%');
+}, 'Specified <percentage> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<resolution>'), '10dpi');
+}, 'Specified <resolution> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<time>'), '1s');
+}, 'Specified <time> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<transform-function>'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Specified <transform-function> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<transform-list>'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Specified <transform-list> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<url>'), 'url("a")');
+}, 'Specified <url> is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<length>+'), '10px 11px');
+}, 'Specified <length>+ is reified as CSSUnparsedValue from get/getAll');
+
+test_specified_maps(function(map){
+    verify_map_get_unparsed(map, generate_property('<length>#'), '10px, 11px');
+}, 'Specified <length># is reified as CSSUnparsedValue from get/getAll');
+
+// StylePropertyMap.set
+
+// The following strings are valid for the specified syntax, and should be
+// accepted by set().
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('*'), 'foo');
+}, 'Specified string "foo" accepted by set() for syntax *');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('foo'), 'foo');
+}, 'Specified string "foo" accepted by set() for syntax foo');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<angle>'), '10deg');
+}, 'Specified string "10deg" accepted by set() for syntax <angle>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<color>'), 'green');
+}, 'Specified string "green" accepted by set() for syntax <color>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<custom-ident>'), 'foo');
+}, 'Specified string "foo" accepted by set() for syntax <custom-ident>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<image>'), 'url("a")');
+}, 'Specified string "url("a")" accepted by set() for syntax <image>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<integer>'), '1');
+}, 'Specified string "1" accepted by set() for syntax <integer>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length-percentage>'), 'calc(10% + 10px)');
+}, 'Specified string "calc(10% + 10px)" accepted by set() for syntax <length-percentage>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>'), '10px');
+}, 'Specified string "10px" accepted by set() for syntax <length>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<number>'), '1');
+}, 'Specified string "1" accepted by set() for syntax <number>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<percentage>'), '10%');
+}, 'Specified string "10%" accepted by set() for syntax <percentage>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<resolution>'), '10dpi');
+}, 'Specified string "10dpi" accepted by set() for syntax <resolution>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<time>'), '1s');
+}, 'Specified string "1s" accepted by set() for syntax <time>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<transform-function>'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Specified string "matrix(0, 0, 0, 0, 0, 0)" accepted by set() for syntax <transform-function>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<transform-list>'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Specified string "matrix(0, 0, 0, 0, 0, 0)" accepted by set() for syntax <transform-list>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<url>'), 'url("a")');
+}, 'Specified string "url("a")" accepted by set() for syntax <url>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>+'), '10px 11px');
+}, 'Specified string "10px 11px" accepted by set() for syntax <length>+');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>#'), '10px, 11px');
+}, 'Specified string "10px, 11px" accepted by set() for syntax <length>#');
+
+// The following strings are invalid for the specified syntax, but should
+// should be accepted by set().
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('foo'), 'bar');
+}, 'Specified string "bar" accepted by set() for syntax foo');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<angle>'), '10px');
+}, 'Specified string "10px" accepted by set() for syntax <angle>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<color>'), '10px');
+}, 'Specified string "10px" accepted by set() for syntax <color>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<custom-ident>'), '10px');
+}, 'Specified string "10px" accepted by set() for syntax <custom-ident>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<image>'), 'a');
+}, 'Specified string "a" accepted by set() for syntax <image>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<integer>'), 'float');
+}, 'Specified string "float" accepted by set() for syntax <integer>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length-percentage>'), 'red');
+}, 'Specified string "red" accepted by set() for syntax <length-percentage>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>'), 'red');
+}, 'Specified string "red" accepted by set() for syntax <length>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<number>'), 'red');
+}, 'Specified string "red" accepted by set() for syntax <number>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<percentage>'), 'var(--x)');
+}, 'Specified string "var(--x)" accepted by set() for syntax <percentage>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<resolution>'), 'blue');
+}, 'Specified string "blue" accepted by set() for syntax <resolution>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<time>'), '1meter');
+}, 'Specified string "1meter" accepted by set() for syntax <time>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<transform-function>'), 'foo(0)');
+}, 'Specified string "foo(0)" accepted by set() for syntax <transform-function>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<transform-list>'), 'bar(1)');
+}, 'Specified string "bar(1)" accepted by set() for syntax <transform-list>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<url>'), 'a');
+}, 'Specified string "a" accepted by set() for syntax <url>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>+'), 'a b');
+}, 'Specified string "a b" accepted by set() for syntax <length>+');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>#'), 'a, b');
+}, 'Specified string "a, b" accepted by set() for syntax <length>#');
+
+// CSSUnparsedValue should always be accepted by any custom property,
+// regardless of registation status.
+
+const unparsed = CSSStyleValue.parse('--x', 'foo bar thing');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('*'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax *');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('foo'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax foo');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<angle>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <angle>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<color>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <color>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<custom-ident>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <custom-ident>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<image>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <image>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<integer>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <integer>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length-percentage>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <length-percentage>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <length>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<number>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <number>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<percentage>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <percentage>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<resolution>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <resolution>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<time>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <time>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<transform-function>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <transform-function>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<transform-list>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <transform-list>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<url>'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <url>');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>+'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <length>+');
+
+test_specified_maps(function(map){
+    verify_map_set(map, generate_property('<length>#'), unparsed);
+}, 'CSSUnparsedValue is accepted via set() for syntax <length>#');
+
+// CSSStyleValues which aren't CSSUnparsedValues aren't accepted by set(),
+// even if they're a value which is compatible with the syntax.
+//
+// https://drafts.css-houdini.org/css-properties-values-api-1/#cssom
+const zero_matrix = CSSStyleValue.parse('transform', 'matrix(0, 0, 0, 0, 0, 0)');
+const image_value = CSSStyleValue.parse('background-image', 'url(a)');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('*'), new CSSKeywordValue('foo'));
+}, 'CSSKeywordValue rejected by set() for syntax *');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('foo'), new CSSKeywordValue('foo'));
+}, 'CSSKeywordValue rejected by set() for syntax foo');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<angle>'), CSS.deg(10));
+}, 'CSSUnitValue rejected by set() for syntax <angle>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<custom-ident>'), new CSSKeywordValue('foo'));
+}, 'CSSKeywordValue rejected by set() for syntax <custom-ident>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<image>'), image_value);
+}, 'CSSImageValue rejected by set() for syntax <image>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<integer>'), CSS.number(1));
+}, 'CSSUnitValue rejected by set() for syntax <integer>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<length-percentage>'), CSS.px(10));
+}, 'CSSUnitValue rejected by set() for syntax <length-percentage>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<length>'), CSS.px(10));
+}, 'CSSUnitValue rejected by set() for syntax <length>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<number>'), CSS.number(10));
+}, 'CSSUnitValue rejected by set() for syntax <number>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<percentage>'), CSS.percent(10));
+}, 'CSSUnitValue rejected by set() for syntax <percentage>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<resolution>'), CSS.dpi(10));
+}, 'CSSUnitValue rejected by set() for syntax <resolution>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<time>'), CSS.s(10));
+}, 'CSSUnitValue rejected by set() for syntax <time>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<transform-list>'), zero_matrix);
+}, 'CSSTransformValue rejected by set() for syntax <transform-list>');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<length>+'), CSS.px(10), CSS.px(10));
+}, 'CSSUnitValue rejected by set() for syntax <length>+');
+
+test_specified_maps(function(map){
+    verify_map_not_set(map, generate_property('<length>#'), CSS.px(10), CSS.px(10));
+}, 'CSSUnitValue rejected by set() for syntax <length>#');
+
+// <color> has no CSSStyleValue subclass yet.
+// <url> has no CSSStyleValue subclass yet.
+// <transform-function> has no CSSStyleValue subclass yet.
+
+// StylePropertyMap.append
+
+// It is not allowed to append CSSStyleValues to custom properties, even
+// when the string matches the syntax of the custom property.
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('*'), 'a');
+}, 'Appending a string to * is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('foo+'), 'foo');
+}, 'Appending a string to foo+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<angle>+'), '10deg');
+}, 'Appending a string to <angle>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<color>+'), 'red');
+}, 'Appending a string to <color>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<custom-ident>+'), 'foo');
+}, 'Appending a string to <custom-ident>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<image>+'), 'url(a)');
+}, 'Appending a string to <image>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<integer>+'), 'a');
+}, 'Appending a string to <integer>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<length-percentage>+'), 'calc(10*% + 10px)');
+}, 'Appending a string to <length-percentage>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<length>+'), '10px');
+}, 'Appending a string to <length>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<number>+'), '1.3');
+}, 'Appending a string to <number>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<percentage>+'), '10%');
+}, 'Appending a string to <percentage>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<resolution>+'), '10dpi');
+}, 'Appending a string to <resolution>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<time>+'), '1s');
+}, 'Appending a string to <time>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<transform-function>+'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Appending a string to <transform-function>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<transform-list>'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Appending a string to <transform-list> is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<url>+'), 'url(a)');
+}, 'Appending a string to <url>+ is not allowed');
+
+test_specified_maps(function(map){
+    verify_map_no_append(map, generate_property('<length>#'), '10px');
+}, 'Appending a string to <length># is not allowed');
+
+// It is not allowed to append CSSStyleValues to custom properties, even
+// when the CSSStyleValue matches the syntax of the custom property.
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('*'), new CSSKeywordValue('foo'));
+}, 'Appending a CSSKeywordValue to * is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('foo+'), new CSSKeywordValue('foo'));
+}, 'Appending a CSSKeywordValue to foo+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<angle>+'), CSS.deg(10));
+}, 'Appending a CSSUnitValue to <angle>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<custom-ident>+'), new CSSKeywordValue('foo'));
+}, 'Appending a CSSKeywordValue to <custom-ident>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<image>+'), image_value);
+}, 'Appending a CSSImageValue to <image>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<integer>+'), CSS.number(1));
+}, 'Appending a CSSUnitValue to <integer>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<length-percentage>+'), CSS.px(10));
+}, 'Appending a CSSUnitValue to <length-percentage>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<length>+'), CSS.px(10));
+}, 'Appending a CSSUnitValue to <length>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<number>+'), CSS.number(10));
+}, 'Appending a CSSUnitValue to <number>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<percentage>+'), CSS.percent(10));
+}, 'Appending a CSSUnitValue to <percentage>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<resolution>+'), CSS.dpi(10));
+}, 'Appending a CSSUnitValue to <resolution>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<time>+'), CSS.s(10));
+}, 'Appending a CSSUnitValue to <time>+ is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<transform-list>'), zero_matrix);
+}, 'Appending a CSSKeywordValue to <transform-list> is not allowed');
+
+test_specified_maps(function(map) {
+    verify_map_no_append(map, generate_property('<length>#'), CSS.px(10));
+}, 'Appending a CSSUnitValue to <length># is not allowed');
+
+// <color> has no CSSStyleValue subclass yet.
+// <url> has no CSSStyleValue subclass yet.
+// <transform-function> has no CSSStyleValue subclass yet.
+
+// CSSStyleValue.parse/parseAll
+
+test(function(){
+    verify_parsed_type(generate_property('*'), 'while(){}', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax *', CSSUnparsedValue);
+
+test(function(){
+    verify_parsed_type(generate_property('<angle>'), '42deg', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <angle>');
+
+test(function(){
+    verify_parsed_type(generate_property('<color>'), '#fefefe', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <color>');
+
+test(function(){
+    verify_parsed_type(generate_property('<custom-ident> | <length>'), 'none', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <custom-ident> | <length>');
+
+test(function(){
+    verify_parsed_type(generate_property('<image>'), 'url(thing.png)', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <image>');
+
+test(function(){
+    verify_parsed_type(generate_property('<integer>'), '100', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <integer>');
+
+test(function(){
+    verify_parsed_type(generate_property('<length-percentage>'), '10%', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <length-percentage> (10%)');
+
+test(function(){
+    verify_parsed_type(generate_property('<length-percentage>'), '10px', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <length-percentage> (10px)');
+
+test(function(){
+    verify_parsed_type(generate_property('<length-percentage>'), 'calc(10px + 10%)', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <length-percentage> (calc(10px + 10%))');
+
+test(function(){
+    verify_parsed_type(generate_property('<length>'), '10px', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <length>');
+
+test(function(){
+    verify_parsed_type(generate_property('<number>'), '42', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <number>');
+
+test(function(){
+    verify_parsed_type(generate_property('<percentage>'), '10%', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <percentage>');
+
+test(function(){
+    verify_parsed_type(generate_property('<resolution>'), '300dpi', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <resolution>');
+
+test(function(){
+    verify_parsed_type(generate_property('<time>'), '42s', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <time>');
+
+test(function(){
+    verify_parsed_type(generate_property('<transform-function>'), 'matrix(0, 0, 0, 0, 0, 0)', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <transform-function>');
+
+test(function(){
+    verify_parsed_type(generate_property('<transform-list>'), 'matrix(0, 0, 0, 0, 0, 0)', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <transform-list>');
+
+test(function(){
+    verify_parsed_type(generate_property('<url>'), 'url(a)', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <url>');
+
+test(function(){
+    verify_parsed_type(generate_property('thing1 | THING2 | <url>'), 'THING2', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax thing1 | THING2 | <url>');
+
+test(function(){
+    verify_parsed_type(generate_property('<length>+'), '10px 20px', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <length>+');
+
+test(function(){
+    verify_parsed_type(generate_property('<length>#'), '10px, 20px', CSSUnparsedValue);
+}, 'CSSStyleValue.parse[All] returns CSSUnparsedValue for syntax <length>#');
+
+// Direct CSSStyleValue objects:
+
+test_specified_maps(function(map){
+    for (let syntax of all_syntaxes()) {
+        let name = generate_property(syntax);
+
+        let initialValue = target.computedStyleMap().get(name);
+
+        // We only care about direct CSSStyleValue instances in this test.
+        // Ultimately, in some future version of CSS TypedOM, we may have no
+        // direct CSSStyleValue instances at all, which is fine.
+        if (initialValue.constructor !== CSSStyleValue) {
+            continue;
+        }
+
+        // Verify that direct CSSStyleValues are rejected by set(). Two things
+        // should prevent this: 1) direct CSSStyleValues are not
+        // CSSUnparsedValues, and 2) direct CSSStyleValues are only valid for
+        // the property they were reified from.
+        verify_map_not_set(map, generate_property(syntax), initialValue);
+    }
+}, 'Direct CSSStyleValue may not be set');
+
+// StylePropertyMap iteration
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('*'), 'foo');
+}, 'Specified * is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('foo'), 'foo');
+}, 'Specified foo is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<angle>'), '10deg');
+}, 'Specified <angle> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<color>'), 'green');
+}, 'Specified <color> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<custom-ident>'), 'foo');
+}, 'Specified <custom-ident> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<image>'), 'url("a")');
+}, 'Specified <image> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<integer>'), '1');
+}, 'Specified <integer> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<length-percentage>'), 'calc(10% + 10px)');
+}, 'Specified <length-percentage> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<length>'), '10px');
+}, 'Specified <length> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<number>'), '1');
+}, 'Specified <number> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<percentage>'), '10%');
+}, 'Specified <percentage> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<resolution>'), '10dpi');
+}, 'Specified <resolution> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<time>'), '1s');
+}, 'Specified <time> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<transform-function>'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Specified <transform-function> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<transform-list>'), 'matrix(0, 0, 0, 0, 0, 0)');
+}, 'Specified <transform-list> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<url>'), 'url("a")');
+}, 'Specified <url> is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<length>+'), '10px 11px');
+}, 'Specified <length>+ is reified CSSUnparsedValue by iterator');
+
+test_specified_maps(function(map){
+    verify_map_iteration_unparsed(map, generate_property('<length>#'), '10px, 11px');
+}, 'Specified <length># is reified CSSUnparsedValue by iterator');
+
+// StylePropertyMapReadOnly iteration
+
+test(function(){
+    let name = generate_property({syntax: '<length>', initialValue: '10px'});
+    let result = Array.from(target.computedStyleMap()).filter(e => e[0] == name)[0];
+    assert_true(typeof(result) !== 'undefined');
+}, 'Registered property with initial value show up on iteration of computedStyleMap');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('*'), 'thing', CSSUnparsedValue);
+}, 'Computed * is reified as CSSUnparsedValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<angle>'), '42deg', CSSUnitValue);
+}, 'Computed <angle> is reified as CSSUnitValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<custom-ident>'), 'thing', CSSKeywordValue);
+}, 'Computed <custom-ident> is reified as CSSKeywordValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<image>'), 'url(\"a\")', CSSImageValue);
+}, 'Computed <image> is reified as CSSImageValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<integer>'), '100', CSSUnitValue);
+}, 'Computed <integer> is reified as CSSUnitValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<length>'), '10px', CSSUnitValue);
+}, 'Computed <length> is reified as CSSUnitValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<number>'), '42', CSSUnitValue);
+}, 'Computed <number> is reified as CSSUnitValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<percentage>'), '10%', CSSUnitValue);
+}, 'Computed <percentage> is reified as CSSUnitValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<resolution>'), '300dppx', CSSUnitValue);
+}, 'Computed <resolution> is reified as CSSUnitValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<time>'), '10s', CSSUnitValue);
+}, 'Computed <time> is reified as CSSUnitValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('none | thing | THING'), 'THING', CSSKeywordValue);
+}, 'Computed none | thing | THING is reified as CSSKeywordValue by iterator');
+
+test(function(){
+    verify_computed_iteration_type(generate_property('<angle> | <length>'), '10px', CSSUnitValue);
+}, 'Computed <angle> | <length> is reified as CSSUnitValue by iterator');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/unit-cycles.html b/testing/web-platform/tests/css/css-properties-values-api/unit-cycles.html
new file mode 100644
index 0000000000..9454c95ac3
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/unit-cycles.html
@@ -0,0 +1,248 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#dependency-cycles-via-relative-units" />
+<meta name="assert" content="This test verifies that reference cycles via units are detected" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+    function register_length(name, inherits=false) {
+        CSS.registerProperty({
+            name: name,
+            syntax: '<length>',
+            initialValue: '0px',
+            inherits: inherits
+        });
+    }
+
+    register_length('--font-size-em');
+    register_length('--font-size-rem');
+    register_length('--font-size-ex');
+    register_length('--font-size-ch');
+    register_length('--font-size-px');
+    register_length('--font-size-lh');
+    register_length('--font-size-em-via-var');
+    register_length('--font-size-rem-via-var');
+    register_length('--font-size-ex-via-var');
+    register_length('--font-size-ch-via-var');
+    register_length('--font-size-em-inherited', true);
+    register_length('--font-size-ex-inherited', true);
+    register_length('--font-size-ch-inherited', true);
+    register_length('--line-height-lh');
+    register_length('--line-height-lh-via-var');
+    register_length('--line-height-lh-inherited', true);
+</script>
+<style>
+    :root {
+        --unregistered-em: 10em;
+        --unregistered-rem: 10rem;
+        --unregistered-ex: 10ex;
+        --unregistered-ch: 10ch;
+        --unregistered-lh: 10lh;
+    }
+
+    :root, #target {
+        --font-size-em: 2em;
+        --font-size-rem: 2rem;
+        --font-size-ex: 2ex;
+        --font-size-ch: 2ch;
+        --font-size-px: 42px;
+        --font-size-lh: 2lh;
+        --line-height-lh: 2lh;
+        --font-size-em-via-var: var(--unregistered-em);
+        --font-size-rem-via-var: var(--unregistered-rem);
+        --font-size-ex-via-var: var(--unregistered-ex);
+        --font-size-ch-via-var: var(--unregistered-ch);
+        --line-height-lh-via-var: var(--unregistered-lh);
+    }
+
+    #parent {
+        --font-size-em-inherited: 4em;
+        --font-size-ex-inherited: 4ex;
+        --font-size-ch-inherited: 4ch;
+        --line-height-lh-inherited: 4lh;
+    }
+
+    #target {
+        font-size: 11px;
+        line-height: 13px;
+    }
+</style>
+
+<div id=parent>
+    <div id=target></div>
+</div>
+<div id=ref></div>
+
+<script>
+
+    // Compute a dimension (e.g. 1em) given a certain fontSize.
+    function compute_dimension(dimension, fontSize, element = ref) {
+        try {
+            element.attributeStyleMap.set('font-size', fontSize);
+            element.attributeStyleMap.set('margin-bottom', dimension);
+            return getComputedStyle(element).marginBottom;
+        } finally {
+            element.attributeStyleMap.clear();
+        }
+    }
+
+    function assert_property_equals(name, value, element = target) {
+        var computedStyle = getComputedStyle(element);
+        assert_equals(computedStyle.getPropertyValue(name), value);
+    }
+
+    let unsetFontSize = compute_dimension('1em', 'unset');
+    const unsetLineHeight = "normal";
+
+    add_result_callback(function(){
+        target.attributeStyleMap.clear();
+        document.documentElement.attributeStyleMap.clear();
+    });
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-px);';
+        assert_property_equals('font-size', '42px');
+        assert_property_equals('--font-size-px', '42px');
+    }, 'Non-font-dependent variables can be used in font-size');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-em);';
+        assert_property_equals('font-size', unsetFontSize);
+        assert_property_equals('--font-size-em', '0px');
+    }, 'Lengths with em units may not be referenced from font-size');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-ex);';
+        assert_property_equals('font-size', unsetFontSize);
+        assert_property_equals('--font-size-ex', '0px');
+    }, 'Lengths with ex units may not be referenced from font-size');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-ch);';
+        assert_property_equals('font-size', unsetFontSize);
+        assert_property_equals('--font-size-ch', '0px');
+    }, 'Lengths with ch units may not be referenced from font-size');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-lh);';
+        assert_property_equals('font-size', unsetFontSize);
+        assert_property_equals('--font-size-lh', '0px');
+    }, 'Lengths with lh units may not be referenced from font-size');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-rem);';
+        let expected = compute_dimension('2rem', 'unset', document.documentElement);
+        assert_property_equals('--font-size-rem', expected);
+        assert_property_equals('font-size', expected);
+    }, 'Lengths with rem units may be referenced from font-size on non-root element');
+
+    test(function() {
+        let root = document.documentElement;
+        root.style = 'font-size: var(--font-size-rem);';
+        assert_property_equals('font-size', unsetFontSize, root);
+        assert_property_equals('--font-size-rem', '0px', root);
+    }, 'Lengths with rem units may not be referenced from font-size on root element');
+
+    test(function() {
+        target.style = 'line-height: var(--line-height-lh);';
+        assert_property_equals('line-height', unsetLineHeight);
+        assert_property_equals('--line-height-lh', '0px');
+    }, 'Lengths with lh units may not be referenced from line-height');
+
+    test(function() {
+        target.style = 'font-size: var(--noexist, var(--font-size-em));';
+        assert_property_equals('font-size', unsetFontSize);
+    }, 'Fallback may not use font-relative units');
+
+    test(function() {
+        target.style = 'line-height: var(--noexist, var(--line-height-lh));';
+        assert_property_equals('line-height', unsetLineHeight);
+    }, 'Fallback may not use line-height-relative units');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-em, 42px);';
+        assert_property_equals('font-size', unsetFontSize);
+    }, 'Fallback not triggered while inside em unit cycle');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-ex, 42px);';
+        assert_property_equals('font-size', unsetFontSize);
+    }, 'Fallback not triggered while inside ex unit cycle');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-ch, 42px);';
+        assert_property_equals('font-size', unsetFontSize);
+    }, 'Fallback not triggered while inside ch unit cycle');
+
+    test(function() {
+        let root = document.documentElement;
+        root.style = 'font-size: var(--font-size-rem, 42px);';
+        assert_property_equals('font-size', unsetFontSize, root);
+        root.style = 'font-size: unset;';
+    }, 'Fallback not triggered while inside rem unit cycle on root element');
+
+    test(function() {
+        target.style = 'line-height: var(--line-height-lh, 42px);';
+        assert_property_equals('line-height', unsetLineHeight);
+    }, 'Fallback not triggered while inside lh unit cycle');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-em-via-var);';
+        assert_property_equals('font-size', unsetFontSize);
+        assert_property_equals('--font-size-em-via-var', '0px');
+    }, 'Lengths with em units are detected via var references');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-ex-via-var);';
+        assert_property_equals('font-size', unsetFontSize);
+        assert_property_equals('--font-size-ex-via-var', '0px');
+    }, 'Lengths with ex units are detected via var references');
+
+    test(function() {
+        target.style = 'font-size: var(--font-size-ch-via-var);';
+        assert_property_equals('font-size', unsetFontSize);
+        assert_property_equals('--font-size-ch-via-var', '0px');
+    }, 'Lengths with ch units are detected via var references');
+
+    test(function() {
+        let root = document.documentElement;
+        root.style = 'font-size: var(--font-size-rem-via-var);';
+        assert_property_equals('font-size', unsetFontSize, root);
+        assert_property_equals('--font-size-rem-via-var', '0px', root);
+        root.style = 'font-size: unset';
+    }, 'Lengths with rem units are detected via var references');
+
+    test(function() {
+        target.style = 'line-height: var(--line-height-lh-via-var);';
+        assert_property_equals('line-height', unsetLineHeight);
+        assert_property_equals('--line-height-lh-via-var', '0px');
+    }, 'Lengths with lh units are detected via var references');
+
+    test(function() {
+        let expected4em = compute_dimension('4em', 'unset');
+        target.style = 'font-size: var(--font-size-em-inherited);';
+        assert_property_equals('font-size', expected4em);
+        assert_property_equals('--font-size-em-inherited', expected4em);
+    }, 'Inherited lengths with em units may be used');
+
+    test(function() {
+        let expected4ex = compute_dimension('4ex', 'unset');
+        target.style = 'font-size: var(--font-size-ex-inherited);';
+        assert_property_equals('font-size', expected4ex);
+        assert_property_equals('--font-size-ex-inherited', expected4ex);
+    }, 'Inherited lengths with ex units may be used');
+
+    test(function() {
+        let expected4ch = compute_dimension('4ch', 'unset');
+        target.style = 'font-size: var(--font-size-ch-inherited);';
+        assert_property_equals('font-size', expected4ch);
+        assert_property_equals('--font-size-ch-inherited', expected4ch);
+    }, 'Inherited lengths with ch units may be used');
+
+    test(function() {
+        let expected4lh = compute_dimension('4lh', 'unset');
+        target.style = 'line-height: var(--line-height-lh-inherited);';
+        assert_property_equals('line-height', expected4lh);
+        assert_property_equals('--line-height-lh-inherited', expected4lh);
+    }, 'Inherited lengths with lh units may be used');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/url-resolution.html b/testing/web-platform/tests/css/css-properties-values-api/url-resolution.html
new file mode 100644
index 0000000000..d932789649
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/url-resolution.html
@@ -0,0 +1,137 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#relative-urls" />
+<meta name="assert" content="This test verifies that relative URLs in registered properties resolve correctly" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<script src="support/main/main.js"></script>
+<script src="support/alt/alt.js"></script>
+<link id="main" rel="stylesheet" type="text/css" href="support/main/main.css" />
+<link id="main_utf16be" rel="stylesheet" type="text/css" href="support/main/main.utf16be.css" />
+<link id="alt" rel="stylesheet" type="text/css" href="support/alt/alt.css" />
+<div id=target>
+    <div id=inner></div>
+</div>
+<script>
+function parse_url(urlstr) {
+    urlstr = urlstr.trim();
+    if (!urlstr.startsWith('url("') || !urlstr.endsWith('")'))
+        throw 'Unknown url format: ' + urlstr;
+    return urlstr.slice(5, -2);
+}
+
+function get_bg_url(element) {
+    return parse_url(getComputedStyle(element)['background-image']);
+}
+
+function get_bg_urls(element) {
+    return getComputedStyle(element)['background-image']
+            .split(',')
+            .map(x => x.trim())
+            .map(x => parse_url(x));
+}
+
+function assert_base_path_equal(actual, expected) {
+    let actual_base = new URL(actual).pathname.split('/').slice(0, -1);
+    let expected_base = new URL(expected).pathname.split('/').slice(0, -1);
+    assert_equals(actual_base.join('/'), expected_base.join('/'));
+}
+
+function assert_base_paths_equal(actual, expected) {
+    assert_equals(actual.length, expected.length);
+    for (let i = 0; i < actual.length; i++) {
+        assert_base_path_equal(actual[i], expected[i]);
+    }
+}
+
+test(function() {
+    target.style = 'background-image: var(--unreg-url);';
+    assert_base_path_equal(get_bg_url(target), document.baseURI);
+}, 'Unregistered property resolves against document (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--unreg-func);';
+    assert_base_path_equal(get_bg_url(target), document.baseURI);
+}, 'Unregistered property resolves against document (URL function)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-non-inherited-url);';
+    assert_base_path_equal(get_bg_url(target), main.sheet.href);
+}, 'Registered non-inherited <url> resolves against sheet (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-non-inherited-func);';
+    assert_base_path_equal(get_bg_url(target), main.sheet.href);
+}, 'Registered non-inherited <url> resolves against sheet (URL function)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-inherited-url);';
+    assert_base_path_equal(get_bg_url(target), main.sheet.href);
+}, 'Registered inherited <url> resolves against sheet (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-inherited-func);';
+    assert_base_path_equal(get_bg_url(target), main.sheet.href);
+}, 'Registered inherited <url> resolves against sheet (URL function)');
+
+test(function() {
+    inner.style = 'background-image: var(--reg-inherited-url);';
+    assert_base_path_equal(get_bg_url(inner), main.sheet.href);
+}, 'Registered inherited <url> resolves against sheet (Child node, URL token)');
+
+test(function() {
+    inner.style = 'background-image: var(--reg-inherited-func);';
+    assert_base_path_equal(get_bg_url(inner), main.sheet.href);
+}, 'Registered inherited <url> resolves against sheet (Child node, URL function)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-ref-to-unreg-url);';
+    assert_base_path_equal(get_bg_url(target), main.sheet.href);
+}, 'Registered property with unregistered var reference resolves against sheet (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-ref-to-unreg-func);';
+    assert_base_path_equal(get_bg_url(target), main.sheet.href);
+}, 'Registered property with unregistered var reference resolves against sheet. (URL function)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-ref-to-reg-url);';
+    assert_base_path_equal(get_bg_url(target), alt.sheet.href);
+}, 'Registered property with registered var reference resolves against sheet of referenced property (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-ref-to-reg-func);';
+    assert_base_path_equal(get_bg_url(target), alt.sheet.href);
+}, 'Registered property with registered var reference resolves against sheet of referenced property (URL function)');
+
+test(function() {
+    target.style = 'background-image: var(--unreg-ref-to-reg-url);';
+    assert_base_path_equal(get_bg_url(target), alt.sheet.href);
+}, 'Unregistered property with registered var reference resolves against sheet of referenced property (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--unreg-ref-to-reg-func);';
+    assert_base_path_equal(get_bg_url(target), alt.sheet.href);
+}, 'Unregistered property with registered var reference resolves against sheet of referenced property (URL function)');
+
+test(function() {
+    target.style = 'background-image: var(--unreg-multi-ref-to-reg-urls);';
+    assert_base_paths_equal(get_bg_urls(target), [main.sheet.href, alt.sheet.href]);
+}, 'Multiple (registered) var reference resolve against respective sheets (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--unreg-multi-ref-to-reg-funcs);';
+    assert_base_paths_equal(get_bg_urls(target), [main.sheet.href, alt.sheet.href]);
+}, 'Multiple (registered) var reference resolve against respective sheets (URL function)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-utf16be-url);';
+    assert_base_path_equal(get_bg_url(target), main_utf16be.sheet.href);
+}, 'Registered UTF16BE-encoded var reference resolve against sheet (URL token)');
+
+test(function() {
+    target.style = 'background-image: var(--reg-utf16be-func);';
+    assert_base_path_equal(get_bg_url(target), main_utf16be.sheet.href);
+}, 'Registered UTF16BE-encoded var reference resolve against sheet (URL function)');
+
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties-cycles.html b/testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties-cycles.html
new file mode 100644
index 0000000000..91792ef51d
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties-cycles.html
@@ -0,0 +1,183 @@
+<!DOCTYPE HTML>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#dom-css-registerproperty" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<style>
+#test1 {
+    --registered-1-a: var(--registered-1-b, 10px);
+    --registered-1-b: var(--registered-1-a, 20px);
+
+    --registered-1-c: var(--registered-1-b, 30px);
+    --registered-1-d: var(--registered-1-b);
+    --unregistered-1-a:var(--registered-1-a,40px);
+    --unregistered-1-a:var(--registered-1-a);
+    left: var(--registered-1-a, 50px);
+    top: var(--registered-1-b, 60px);
+}
+</style>
+<div id=test1></div>
+<script>
+test(function() {
+    CSS.registerProperty({name: '--registered-1-a', syntax: '<length>', initialValue: '1px', inherits: false});
+    CSS.registerProperty({name: '--registered-1-b', syntax: '<length>', initialValue: '2px', inherits: false});
+    CSS.registerProperty({name: '--registered-1-c', syntax: '<length>', initialValue: '3px', inherits: false});
+    CSS.registerProperty({name: '--registered-1-d', syntax: '<length>', initialValue: '4px', inherits: false});
+
+    computedStyle = getComputedStyle(test1);
+    assert_equals(computedStyle.getPropertyValue('--registered-1-a'), '1px');
+    assert_equals(computedStyle.getPropertyValue('--registered-1-b'), '2px');
+    assert_equals(computedStyle.getPropertyValue('--registered-1-c'), '2px');
+    assert_equals(computedStyle.getPropertyValue('--registered-1-d'), '2px');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-1-a'), '1px');
+    assert_equals(computedStyle.left, '1px');
+    assert_equals(computedStyle.top, '2px');
+}, "A var() cycle between two registered properties is handled correctly.");
+</script>
+
+<style>
+#test2 {
+    --registered-2-a: var(--unregistered-2-a, 10px);
+    --unregistered-2-a:var(--registered-2-a,20px);
+
+    --registered-2-b: var(--registered-2-a, 30px);
+    --registered-2-c: var(--registered-2-a);
+    --registered-2-d: var(--unregistered-2-a, 40px);
+    --registered-2-e: var(--unregistered-2-a);
+    --unregistered-2-b:var(--registered-2-a,50px);
+    --unregistered-2-c:var(--registered-2-a);
+    --unregistered-2-d:var(--unregistered-2-a,60px);
+    --unregistered-2-e:var(--unregistered-2-a);
+    left: var(--registered-2-a, 70px);
+    top: var(--unregistered-2-a, 80px);
+}
+</style>
+<div id=test2></div>
+<script>
+test(function() {
+    CSS.registerProperty({name: '--registered-2-a', syntax: '<length>', initialValue: '1px', inherits: false});
+    CSS.registerProperty({name: '--registered-2-b', syntax: '<length>', initialValue: '2px', inherits: false});
+    CSS.registerProperty({name: '--registered-2-c', syntax: '<length>', initialValue: '3px', inherits: false});
+    CSS.registerProperty({name: '--registered-2-d', syntax: '<length>', initialValue: '4px', inherits: false});
+    CSS.registerProperty({name: '--registered-2-e', syntax: '<length>', initialValue: '5px', inherits: false});
+
+    computedStyle = getComputedStyle(test2);
+    assert_equals(computedStyle.getPropertyValue('--registered-2-a'), '1px');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-2-a'), '');
+
+    assert_equals(computedStyle.getPropertyValue('--registered-2-b'), '1px');
+    assert_equals(computedStyle.getPropertyValue('--registered-2-c'), '1px');
+    assert_equals(computedStyle.getPropertyValue('--registered-2-d'), '40px');
+    assert_equals(computedStyle.getPropertyValue('--registered-2-e'), '5px');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-2-b'), '1px');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-2-c'), '1px');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-2-d'), '60px');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-2-e'), '');
+    assert_equals(computedStyle.left, '1px');
+    assert_equals(computedStyle.top, '80px');
+}, "A var() cycle between a registered properties and an unregistered property is handled correctly.");
+</script>
+
+<style>
+#test3 {
+    --unregistered-3-a:var(--unregistered-3-b,10px);
+    --unregistered-3-b:var(--unregistered-3-a,20px);
+
+    --registered-3-a: var(--unregistered-3-a, 30px);
+    --registered-3-b: var(--unregistered-3-a);
+    --registered-3-c: var(--unregistered-3-b, 40px);
+    --registered-3-d: var(--registered-3-c, 50px);
+    left: var(--registered-3-d, 60px);
+    top: var(--registered-3-b, 70px);
+}
+</style>
+<div id=test3></div>
+<script>
+test(function() {
+    CSS.registerProperty({name: '--registered-3-a', syntax: '<length>', initialValue: '1px', inherits: false});
+    CSS.registerProperty({name: '--registered-3-b', syntax: '<length>', initialValue: '2px', inherits: false});
+    CSS.registerProperty({name: '--registered-3-c', syntax: '<length>', initialValue: '3px', inherits: false});
+    CSS.registerProperty({name: '--registered-3-d', syntax: '<length>', initialValue: '4px', inherits: false});
+
+    computedStyle = getComputedStyle(test3);
+    assert_equals(computedStyle.getPropertyValue('--unregistered-3-a'), '');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-3-b'), '');
+
+    assert_equals(computedStyle.getPropertyValue('--registered-3-a'), '30px');
+    assert_equals(computedStyle.getPropertyValue('--registered-3-b'), '2px');
+    assert_equals(computedStyle.getPropertyValue('--registered-3-c'), '40px');
+    assert_equals(computedStyle.getPropertyValue('--registered-3-d'), '40px');
+    assert_equals(computedStyle.left, '40px');
+    assert_equals(computedStyle.top, '2px');
+}, "A var() cycle between a two unregistered properties is handled correctly.");
+</script>
+
+<style>
+#test4 {
+    --registered-4-a:var(--unregistered-4-a,hello);
+    --unregistered-4-a:var(--registered-4-a,world);
+
+    --registered-4-b:var(--unregistered-4-a,meow);
+    --registered-4-c:var(--unregistered-4-a);
+    --unregistered-4-b:var(--unregistered-4-a,woof);
+    --unregistered-4-c:var(--unregistered-4-a);
+    transition-property: var(--registered-4-a, water);
+}
+</style>
+<div id=test4></div>
+<script>
+test(function() {
+    CSS.registerProperty({name: '--registered-4-a', syntax: '*', inherits: false});
+    CSS.registerProperty({name: '--registered-4-b', syntax: '*', initialValue: 'moo', inherits: false});
+    CSS.registerProperty({name: '--registered-4-c', syntax: '*', initialValue: 'circle', inherits: false});
+
+    computedStyle = getComputedStyle(test4);
+    assert_equals(computedStyle.getPropertyValue('--registered-4-a'), '');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-4-a'), '');
+
+    assert_equals(computedStyle.getPropertyValue('--registered-4-b'), 'meow');
+    assert_equals(computedStyle.getPropertyValue('--registered-4-c'), '');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-4-b'), 'woof');
+    assert_equals(computedStyle.getPropertyValue('--unregistered-4-c'), '');
+    assert_equals(computedStyle.transitionProperty, 'water');
+}, "A var() cycle between a syntax:'*' property and an unregistered property is handled correctly.");
+</script>
+
+<style>
+#test5_parent {
+    --registered-5-c:foo;
+    --registered-5-d:bar;
+    --registered-5-e:baz;
+    color: green;
+}
+#test5 {
+    --registered-5-a:var(--registered-5-b,hello);
+    --registered-5-b:var(--registered-5-a,world);
+
+    --registered-5-c:var(--registered-5-a);
+    --registered-5-d:var(--registered-5-b);
+    --registered-5-e:var(--unknown);
+    color: var(--registered-5-e);
+}
+</style>
+<div id=test5_parent>
+    <div id=test5></div>
+</div>
+<script>
+test(function() {
+    CSS.registerProperty({name: '--registered-5-a', syntax: '*', inherits: true});
+    CSS.registerProperty({name: '--registered-5-b', syntax: '*', inherits: true});
+    CSS.registerProperty({name: '--registered-5-c', syntax: '*', inherits: true});
+    CSS.registerProperty({name: '--registered-5-d', syntax: '*', inherits: true});
+    CSS.registerProperty({name: '--registered-5-e', syntax: '*', inherits: true});
+
+    let computedStyle = getComputedStyle(test5);
+    assert_equals(computedStyle.getPropertyValue('--registered-5-a'), '');
+    assert_equals(computedStyle.getPropertyValue('--registered-5-b'), '');
+    assert_equals(computedStyle.getPropertyValue('--registered-5-c'), '');
+    assert_equals(computedStyle.getPropertyValue('--registered-5-d'), '');
+    assert_equals(computedStyle.getPropertyValue('--registered-5-e'), '');
+    assert_equals(computedStyle.getPropertyValue('color'), 'rgb(0, 128, 0)');
+}, "Custom properties with universal syntax become guaranteed-invalid when " +
+        "invalid at computed-value time");
+</script>
diff --git a/testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties.html b/testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties.html
new file mode 100644
index 0000000000..8adfb09666
--- /dev/null
+++ b/testing/web-platform/tests/css/css-properties-values-api/var-reference-registered-properties.html
@@ -0,0 +1,182 @@
+<!DOCTYPE HTML>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api/#dom-css-registerproperty" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/utils.js"></script>
+<style>
+div {
+    --registered-length-1: 10px;
+    --registered-length-2: var(--registered-length-1);
+    --registered-length-3: var(--length-1);
+    --registered-length-4: calc(var(--length-1) + 40px);
+    --registered-length-5: var(--invalid, 70px);
+    --registered-length-6: calc(var(--registered-length-3)*4);
+    --registered-length-7: var(--123px, 6px);
+
+    --length-1: 20px;
+    --length-2: var(--registered-length-1);
+    --length-3: calc(var(--123px, 6px) + var(--123px));
+
+    --percentage: 10%;
+    --registered-length-invalid: var(--percentage);
+
+    --registered-token-stream-1:var(--invalid);
+    --registered-token-stream-2:var(--invalid,fallback);
+    --token-stream-1:var(--registered-token-stream-1,moo);
+
+    --registered-length-list-1: 1px, var(--registered-length-1), 2px;
+    --registered-length-list-2: 1px, var(--length-1), var(--registered-length-1), 2px;
+    --registered-length-list-3: var(--registered-length-list-1), var(--registered-length-list-2);
+}
+</style>
+<div id=element></div>
+<script>
+test(function() {
+    CSS.registerProperty({name: '--123px', syntax: '<length>', initialValue: '123px', inherits: false});
+
+    CSS.registerProperty({name: '--registered-length-1', syntax: '<length>', initialValue: '0px', inherits: false});
+    CSS.registerProperty({name: '--registered-length-2', syntax: '<length>', initialValue: '0px', inherits: false});
+    CSS.registerProperty({name: '--registered-length-3', syntax: '<length>', initialValue: '0px', inherits: false});
+    CSS.registerProperty({name: '--registered-length-4', syntax: '<length>', initialValue: '0px', inherits: false});
+    CSS.registerProperty({name: '--registered-length-5', syntax: '<length>', initialValue: '0px', inherits: false});
+    CSS.registerProperty({name: '--registered-length-6', syntax: '<length>', initialValue: '0px', inherits: false});
+    CSS.registerProperty({name: '--registered-length-7', syntax: '<length>', initialValue: '0px', inherits: false});
+    CSS.registerProperty({name: '--registered-length-invalid', syntax: '<length>', initialValue: '15px', inherits: false});
+
+    CSS.registerProperty({name: '--registered-token-stream-1', syntax: '*', inherits: false});
+    CSS.registerProperty({name: '--registered-token-stream-2', syntax: '*', inherits: false});
+
+    computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue('--registered-length-1'), '10px');
+    assert_equals(computedStyle.getPropertyValue('--registered-length-2'), '10px');
+    assert_equals(computedStyle.getPropertyValue('--registered-length-3'), '20px');
+    assert_equals(computedStyle.getPropertyValue('--registered-length-4'), '60px');
+    assert_equals(computedStyle.getPropertyValue('--registered-length-5'), '70px');
+    assert_equals(computedStyle.getPropertyValue('--registered-length-6'), '80px');
+    assert_equals(computedStyle.getPropertyValue('--registered-length-7'), '123px');
+    assert_equals(computedStyle.getPropertyValue('--length-1'), '20px');
+    assert_equals(computedStyle.getPropertyValue('--length-2'), '10px');
+    assert_equals(computedStyle.getPropertyValue('--length-3'), 'calc(123px + 123px)');
+    assert_equals(computedStyle.getPropertyValue('--registered-length-invalid'), '15px');
+
+    assert_equals(computedStyle.getPropertyValue('--registered-token-stream-1'), '');
+    assert_equals(computedStyle.getPropertyValue('--registered-token-stream-2'), 'fallback');
+    assert_equals(computedStyle.getPropertyValue('--token-stream-1'), 'moo');
+}, "var() references work with registered properties");
+
+test(function(){
+    CSS.registerProperty({
+        name: '--registered-length-list-1',
+        syntax: '<length>#',
+        initialValue: '0px',
+        inherits: false
+    });
+    let computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue('--registered-length-list-1'), '1px, 10px, 2px');
+}, 'References to registered var()-properties work in registered lists');
+
+test(function(){
+    CSS.registerProperty({
+        name: '--registered-length-list-2',
+        syntax: '<length>#',
+        initialValue: '0px',
+        inherits: false
+    });
+    let computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue('--registered-length-list-2'), '1px, 20px, 10px, 2px');
+}, 'References to mixed registered and unregistered var()-properties work in registered lists');
+
+test(function(){
+    CSS.registerProperty({
+        name: '--registered-length-list-3',
+        syntax: '<length>#',
+        initialValue: '0px',
+        inherits: false
+    });
+    let computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue('--registered-length-list-3'), '1px, 10px, 2px, 1px, 20px, 10px, 2px');
+}, 'Registered lists may be concatenated');
+
+test(function(){
+    CSS.registerProperty({
+        name: '--length-em',
+        syntax: '<length>',
+        initialValue: '0px',
+        inherits: false
+    });
+    element.style = 'font-size: 11px; --length-em: 10em; --unregistered:var(--length-em);';
+    let computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue('--unregistered'), '110px');
+    element.style = '';
+}, 'Font-relative units are absolutized when substituting');
+
+test(function(){
+    CSS.registerProperty({
+        name: '--length-calc',
+        syntax: '<length>',
+        initialValue: '0px',
+        inherits: false
+    });
+    element.style = 'font-size: 11px; --length-calc: calc(10em + 10px); --unregistered:var(--length-calc);';
+    let computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue('--unregistered'), '120px');
+    element.style = '';
+}, 'Calc expressions are resolved when substituting');
+
+test(function(){
+    CSS.registerProperty({
+        name: '--length-calc-list',
+        syntax: '<length>#',
+        initialValue: '0px',
+        inherits: false
+    });
+    element.style = 'font-size: 11px; --length-calc-list: 10em, calc(10em + 10px); --unregistered:var(--length-calc-list);';
+    let computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue('--unregistered'), '110px, 120px');
+    element.style = '';
+}, 'Lists with relative units are absolutized when substituting');
+
+test(function(){
+    let length = generate_property('none | <length>');
+    let universal = generate_property('*');
+    element.style = `font-size: 10px; ${length}: 10em; ${universal}: var(${length})`;
+    let computedStyle = getComputedStyle(element);
+    assert_equals(computedStyle.getPropertyValue(universal), ' 100px');
+    element.style = '';
+}, 'Values are absolutized when substituting into properties with universal syntax');
+
+function test_valid_fallback(syntax, value, fallback) {
+    test(function(){
+        let name = generate_property(syntax);
+        try {
+            element.style = `${name}: ${value}; --x:var(${name},${fallback})`;
+            let computedStyle = getComputedStyle(element);
+            assert_equals(computedStyle.getPropertyValue('--x'), value);
+        } finally {
+            element.style = '';
+        }
+    }, `Valid fallback does not invalidate var()-reference [${syntax}, ${fallback}]`);
+}
+
+function test_invalid_fallback(syntax, value, fallback) {
+    test(function(){
+        let name = generate_property(syntax);
+        try {
+            element.style = `${name}: ${value}; --x:var(${name},${fallback})`;
+            let computedStyle = getComputedStyle(element);
+            assert_equals(computedStyle.getPropertyValue('--x'), '');
+        } finally {
+            element.style = '';
+        }
+    }, `Invalid fallback invalidates var()-reference [${syntax}, ${fallback}]`);
+}
+
+test_valid_fallback('<length>', '40px', '10px');
+test_valid_fallback('<length> | <color>', '40px', 'red');
+test_valid_fallback('<length> | none', '40px', 'none');
+
+test_invalid_fallback('<length>', '40px', 'red');
+test_invalid_fallback('<length> | none', '40px', 'nolength');
+test_invalid_fallback('<length>', '40px', 'var(--novar)');
+
+</script>
-- 
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