From 544a340956293550ca5eeb89d7a046879527df18 Mon Sep 17 00:00:00 2001
From: Robert Flack <flackr@chromium.org>
Date: Wed, 07 Oct 2026 06:30:41 -0700
Subject: [PATCH] Fix pre-interpolation composition with mismatched underlying values

When converting non-neutral additive or accumulative keyframes,
InvalidatableInterpolation::MaybeConvertPairwise and
ConvertSingleKeyframe should only pass underlying_value_owner.Value() to
with a matching InterpolationType.

This fixes issues when animating a registered custom property whose
underlying value may be absent, or not convertible by any
InterpolationType, or produced by a different InterpolationType.

Bug: 566496747
Change-Id: Ide25c19ff4a63c8e000f90c26fcd4dc1a5b7bc65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8521821
Commit-Queue: Robert Flack <flackr@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1713374}
---
diff --git a/third_party/blink/renderer/core/animation/css_custom_list_interpolation_type.cc b/third_party/blink/renderer/core/animation/css_custom_list_interpolation_type.cc
index b3fda63..cdf44e25 100644
--- a/third_party/blink/renderer/core/animation/css_custom_list_interpolation_type.cc
+++ b/third_party/blink/renderer/core/animation/css_custom_list_interpolation_type.cc
@@ -73,6 +73,12 @@
     const InterpolationValue& underlying,
     EffectModel::CompositeOperation composite,
     ConversionCheckers& conversion_checkers) const {
+  conversion_checkers.push_back(MakeGarbageCollected<UnderlyingLengthChecker>(
+      UnderlyingLengthChecker::GetUnderlyingLength(underlying)));
+  if (!underlying) {
+    return value;
+  }
+
   // This adapts a ListInterpolationFunctions::CompositeItemCallback function
   // such that we can use the InterpolationType::Composite function of the
   // inner interpolation type to get the answer.
@@ -82,12 +88,10 @@
   UnderlyingValueOwner owner;
   owner.Set(this, underlying);
 
-  ConversionCheckers null_checkers;
-
   const CSSInterpolationType* interpolation_type =
       inner_interpolation_type_.Get();
   auto composite_callback =
-      [interpolation_type, composite, &null_checkers](
+      [interpolation_type, composite, &conversion_checkers](
           UnderlyingValue& underlying_value, double underlying_fraction,
           const InterpolableValue& interpolable_value,
           const NonInterpolableValue* non_interpolable_value) {
@@ -99,7 +103,7 @@
             underlying_value.GetNonInterpolableValue());
         InterpolationValue composite_result =
             interpolation_type->PreInterpolationCompositeIfNeeded(
-                std::move(value), underlying, composite, null_checkers);
+                std::move(value), underlying, composite, conversion_checkers);
         composite_result = composite_result.Clone();
         underlying_value.SetInterpolableValue(
             composite_result.interpolable_value);
diff --git a/third_party/blink/renderer/core/animation/css_transform_interpolation_type.cc b/third_party/blink/renderer/core/animation/css_transform_interpolation_type.cc
index 860bf75..3fc0f43c 100644
--- a/third_party/blink/renderer/core/animation/css_transform_interpolation_type.cc
+++ b/third_party/blink/renderer/core/animation/css_transform_interpolation_type.cc
@@ -156,6 +156,10 @@
   conversion_checkers.push_back(
       MakeGarbageCollected<AlwaysInvalidateChecker>());
 
+  if (!underlying) {
+    return value;
+  }
+
   InterpolableTransformList& transform_list =
       To<InterpolableTransformList>(*value.interpolable_value);
   const InterpolableTransformList& underlying_transform_list =
diff --git a/third_party/blink/renderer/core/animation/invalidatable_interpolation.cc b/third_party/blink/renderer/core/animation/invalidatable_interpolation.cc
index 7f46d93..b757be9 100644
--- a/third_party/blink/renderer/core/animation/invalidatable_interpolation.cc
+++ b/third_party/blink/renderer/core/animation/invalidatable_interpolation.cc
@@ -42,17 +42,23 @@
 InvalidatableInterpolation::MaybeConvertPairwise(
     const CSSInterpolationEnvironment& environment,
     const UnderlyingValueOwner& underlying_value_owner) const {
+  const InterpolationValue null_underlying(nullptr);
   for (const InterpolationType* interpolation_type : *interpolation_types_) {
     if ((start_keyframe_->IsNeutral() || end_keyframe_->IsNeutral()) &&
         (!underlying_value_owner ||
          underlying_value_owner.GetType() != interpolation_type)) {
       continue;
     }
+    const InterpolationValue& underlying =
+        (underlying_value_owner &&
+         underlying_value_owner.GetType() == interpolation_type)
+            ? underlying_value_owner.Value()
+            : null_underlying;
     ConversionCheckers conversion_checkers;
     PairwiseInterpolationValue result =
         interpolation_type->MaybeConvertPairwise(
-            *start_keyframe_, *end_keyframe_, environment,
-            underlying_value_owner.Value(), conversion_checkers);
+            *start_keyframe_, *end_keyframe_, environment, underlying,
+            conversion_checkers);
     AddConversionCheckers(interpolation_type, conversion_checkers);
     if (result) {
       return MakeGarbageCollected<PairwisePrimitiveInterpolation>(
@@ -71,15 +77,20 @@
   if (keyframe.IsNeutral() && !underlying_value_owner) {
     return nullptr;
   }
+  const InterpolationValue null_underlying(nullptr);
   for (const InterpolationType* interpolation_type : *interpolation_types_) {
     if (keyframe.IsNeutral() &&
         underlying_value_owner.GetType() != interpolation_type) {
       continue;
     }
+    const InterpolationValue& underlying =
+        (underlying_value_owner &&
+         underlying_value_owner.GetType() == interpolation_type)
+            ? underlying_value_owner.Value()
+            : null_underlying;
     ConversionCheckers conversion_checkers;
     InterpolationValue result = interpolation_type->MaybeConvertSingle(
-        keyframe, environment, underlying_value_owner.Value(),
-        conversion_checkers);
+        keyframe, environment, underlying, conversion_checkers);
     AddConversionCheckers(interpolation_type, conversion_checkers);
     if (result) {
       return MakeGarbageCollected<TypedInterpolationValue>(interpolation_type,
@@ -154,8 +165,14 @@
       return false;
     }
   }
+  const InterpolationValue null_underlying(nullptr);
   for (const auto& checker : conversion_checkers_) {
-    if (!checker->IsValid(environment, underlying_value_owner.Value())) {
+    const InterpolationValue& underlying =
+        (underlying_value_owner &&
+         underlying_value_owner.GetType() == checker->GetType())
+            ? underlying_value_owner.Value()
+            : null_underlying;
+    if (!checker->IsValid(environment, underlying)) {
       return false;
     }
   }
diff --git a/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html b/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html
index f8e0a01..0ef9f7c3 100644
--- a/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html
+++ b/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html
@@ -144,5 +144,70 @@
   {at: 1.5, is: '100px'},
 ]);
 
+CSS.registerProperty({
+  name: '--transform-list-or-none',
+  syntax: '<transform-list> | none',
+  initialValue: 'none',
+  inherits: false
+});
+
+CSS.registerProperty({
+  name: '--number-list-or-transform-list',
+  syntax: '<number>+ | <transform-list>',
+  initialValue: '100',
+  inherits: false
+});
+
+CSS.registerProperty({
+  name: '--length-list-or-transform-function-list',
+  syntax: '<length># | <transform-function>#',
+  initialValue: '10px, 20px',
+  inherits: false
+});
+
+assertComposition({
+  property: '--transform-list-or-none',
+  underlying: 'none',
+  addFrom: 'translateX(10px) scale(2)',
+  addTo: 'translateX(100px) scale(4)',
+}, [
+  {at: 0, is: 'translateX(10px) scale(2)'},
+  {at: 0.5, is: 'translateX(55px) scale(3)'},
+  {at: 1, is: 'translateX(100px) scale(4)'},
+]);
+
+assertComposition({
+  property: '--number-list-or-transform-list',
+  underlying: '100',
+  addFrom: 'translateX(10px)',
+  addTo: 'translateX(100px)',
+}, [
+  {at: 0, is: 'translateX(10px)'},
+  {at: 0.5, is: 'translateX(55px)'},
+  {at: 1, is: 'translateX(100px)'},
+]);
+
+assertComposition({
+  property: '--number-list-or-transform-list',
+  underlying: 'translateX(100px)',
+  addFrom: '10',
+  addTo: '100',
+}, [
+  {at: 0, is: '10'},
+  {at: 0.5, is: '55'},
+  {at: 1, is: '100'},
+]);
+
+assertComposition({
+  property: '--length-list-or-transform-function-list',
+  underlying: '10px, 20px',
+  addFrom: 'translateX(10px), scale(2)',
+  addTo: 'translateX(100px), scale(4)',
+}, [
+  {at: 0, is: 'translateX(10px), scale(2)'},
+  {at: 0.5, is: 'translateX(55px), scale(3)'},
+  {at: 1, is: 'translateX(100px), scale(4)'},
+]);
+
 </script>
 </body>
diff --git a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html
index 9575744..3f0fc5bf 100644
--- a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html
+++ b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html
@@ -15,4 +15,88 @@
   expected: "200"
 }, 'Animating a custom property allowing multiple list types with two mismatching types');
 
+animation_test({
+  syntax: "<number>+ | <transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  composite: "add",
+  keyframes: ["100", "200"],
+  expected: "150"
+}, 'Additive animation of <number>+ when underlying value is <transform-list>');
+
+animation_test({
+  syntax: "<number>+ | <transform-list>",
+  inherits: false,
+  initialValue: "100"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px)", "translateX(200px)"],
+  expected: "translateX(150px)"
+}, 'Additive animation of <transform-list> when underlying value is <number>+');
+
+animation_test({
+  syntax: "<transform-function> | <transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px) scale(2)"
+}, {
+  composite: "add",
+  keyframes: ["translateX(200px)", "translateX(300px)"],
+  expected: "translateX(250px)"
+}, 'Additive animation of <transform-function> when underlying value is a multi-item <transform-list>');
+
+animation_test({
+  syntax: "<length># | <transform-function>#",
+  inherits: false,
+  initialValue: "10px, 20px"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px), scale(2)", "translateX(200px), scale(4)"],
+  expected: "translateX(150px), scale(3)"
+}, 'Additive animation of <transform-function># when underlying value is <length>#');
+
+animation_test({
+  syntax: "<transform-function># | none",
+  inherits: false,
+  initialValue: "none"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px), scale(2)", "translateX(200px), scale(4)"],
+  expected: "translateX(150px), scale(3)"
+}, 'Additive animation of <transform-function># when underlying value is none');
+
+test(() => {
+  const name = generate_name();
+  CSS.registerProperty({
+    name,
+    syntax: "<transform-function>#",
+    inherits: false,
+    initialValue: "translateX(10px), scale(2)"
+  });
+
+  target.style.setProperty(name, "translateX(10px)");
+  const anim = target.animate(
+    { [name]: ["translateX(20px), scale(3)", "translateX(40px), scale(5)"] },
+    { duration: 1000, composite: "add" }
+  );
+  anim.pause();
+  anim.currentTime = 500;
+
+  // Underlying list length (1) != keyframe list length (2), falls back to replace.
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(30px), scale(4)"
+  );
+
+  // Dynamically updating underlying list to length 2 invalidates cache and composites.
+  target.style.setProperty(name, "translateX(10px), scale(2)");
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(40px), scale(5)"
+  );
+
+  target.style.removeProperty(name);
+  anim.cancel();
+}, 'Additive <transform-function># animation with dynamic underlying list length and value update');
+
 </script>
diff --git a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
index 161703a6..f090251 100644
--- a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
+++ b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
@@ -65,4 +65,123 @@
   assert_function: assert_matrix_equals
 }, 'Animating a custom property of type <transform-list> containing multiple values and with mismatching list lengths');
 
+animation_test({
+  syntax: "<transform-list> | none",
+  inherits: false,
+  initialValue: "none"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px) scale(2)", "translateX(200px) scale(4)"],
+  expected: "translateX(150px) scale(3)"
+}, 'Animating a custom property of type <transform-list> | none with additivity when underlying value is none');
+
+animation_test({
+  syntax: "<transform-list> | none",
+  inherits: false,
+  initialValue: "none"
+}, {
+  composite: "accumulate",
+  keyframes: ["translateX(100px) scale(2)", "translateX(200px) scale(4)"],
+  expected: "translateX(150px) scale(3)"
+}, 'Animating a custom property of type <transform-list> | none with accumulation when underlying value is none');
+
+test(() => {
+  const name = generate_name();
+  CSS.registerProperty({
+    name,
+    syntax: "<transform-list> | none",
+    inherits: false,
+    initialValue: "translateX(100px) scale(2)"
+  });
+
+  target.style.setProperty(name, "none");
+  const animation = target.animate(
+    { [name]: ["translateX(200px) scale(3)", "translateX(300px) scale(4)"] },
+    { duration: 1000, composite: "add" }
+  );
+  animation.pause();
+  animation.currentTime = 500;
+
+  // When the underlying value is none, composition falls back to replace.
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(250px) scale(3.5)"
+  );
+
+  // Dynamically restoring a valid underlying <transform-list> invalidates the
+  // conversion cache and composites with the new underlying value.
+  target.style.setProperty(name, "translateX(100px) scale(2)");
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(100px) scale(2) translateX(250px) scale(3.5)"
+  );
+
+  target.style.removeProperty(name);
+  animation.cancel();
+}, 'Additive <transform-list> | none animation with dynamic underlying value update from none');
+
+test(() => {
+  const name = generate_name();
+  CSS.registerProperty({
+    name,
+    syntax: "<transform-list>",
+    inherits: false,
+    initialValue: "translateX(100px) scale(2)"
+  });
+
+  const baseAnim = target.animate(
+    { [name]: ["var(--invalid, invalid)", "var(--invalid, invalid)"] },
+    { duration: 1000 }
+  );
+  baseAnim.pause();
+  baseAnim.currentTime = 500;
+
+  const addAnim = target.animate(
+    { [name]: ["translateX(200px) scale(3)", "translateX(300px) scale(4)"] },
+    { duration: 1000, composite: "add" }
+  );
+  addAnim.pause();
+  addAnim.currentTime = 500;
+
+  assert_equals(
... (file diff truncated)

diff --git a/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html b/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html
index f8e0a01..0ef9f7c3 100644
--- a/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html
+++ b/third_party/blink/web_tests/animations/custom-properties/custom-list-type-interpolation.html
@@ -144,5 +144,70 @@
   {at: 1.5, is: '100px'},
 ]);
 
+CSS.registerProperty({
+  name: '--transform-list-or-none',
+  syntax: '<transform-list> | none',
+  initialValue: 'none',
+  inherits: false
+});
+
+CSS.registerProperty({
+  name: '--number-list-or-transform-list',
+  syntax: '<number>+ | <transform-list>',
+  initialValue: '100',
+  inherits: false
+});
+
+CSS.registerProperty({
+  name: '--length-list-or-transform-function-list',
+  syntax: '<length># | <transform-function>#',
+  initialValue: '10px, 20px',
+  inherits: false
+});
+
+assertComposition({
+  property: '--transform-list-or-none',
+  underlying: 'none',
+  addFrom: 'translateX(10px) scale(2)',
+  addTo: 'translateX(100px) scale(4)',
+}, [
+  {at: 0, is: 'translateX(10px) scale(2)'},
+  {at: 0.5, is: 'translateX(55px) scale(3)'},
+  {at: 1, is: 'translateX(100px) scale(4)'},
+]);
+
+assertComposition({
+  property: '--number-list-or-transform-list',
+  underlying: '100',
+  addFrom: 'translateX(10px)',
+  addTo: 'translateX(100px)',
+}, [
+  {at: 0, is: 'translateX(10px)'},
+  {at: 0.5, is: 'translateX(55px)'},
+  {at: 1, is: 'translateX(100px)'},
+]);
+
+assertComposition({
+  property: '--number-list-or-transform-list',
+  underlying: 'translateX(100px)',
+  addFrom: '10',
+  addTo: '100',
+}, [
+  {at: 0, is: '10'},
+  {at: 0.5, is: '55'},
+  {at: 1, is: '100'},
+]);
+
+assertComposition({
+  property: '--length-list-or-transform-function-list',
+  underlying: '10px, 20px',
+  addFrom: 'translateX(10px), scale(2)',
+  addTo: 'translateX(100px), scale(4)',
+}, [
+  {at: 0, is: 'translateX(10px), scale(2)'},
+  {at: 0.5, is: 'translateX(55px), scale(3)'},
+  {at: 1, is: 'translateX(100px), scale(4)'},
+]);
+
 </script>
 </body>
diff --git a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html
index 9575744..3f0fc5bf 100644
--- a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html
+++ b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-list-type-mismatch.html
@@ -15,4 +15,88 @@
   expected: "200"
 }, 'Animating a custom property allowing multiple list types with two mismatching types');
 
+animation_test({
+  syntax: "<number>+ | <transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px)"
+}, {
+  composite: "add",
+  keyframes: ["100", "200"],
+  expected: "150"
+}, 'Additive animation of <number>+ when underlying value is <transform-list>');
+
+animation_test({
+  syntax: "<number>+ | <transform-list>",
+  inherits: false,
+  initialValue: "100"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px)", "translateX(200px)"],
+  expected: "translateX(150px)"
+}, 'Additive animation of <transform-list> when underlying value is <number>+');
+
+animation_test({
+  syntax: "<transform-function> | <transform-list>",
+  inherits: false,
+  initialValue: "translateX(100px) scale(2)"
+}, {
+  composite: "add",
+  keyframes: ["translateX(200px)", "translateX(300px)"],
+  expected: "translateX(250px)"
+}, 'Additive animation of <transform-function> when underlying value is a multi-item <transform-list>');
+
+animation_test({
+  syntax: "<length># | <transform-function>#",
+  inherits: false,
+  initialValue: "10px, 20px"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px), scale(2)", "translateX(200px), scale(4)"],
+  expected: "translateX(150px), scale(3)"
+}, 'Additive animation of <transform-function># when underlying value is <length>#');
+
+animation_test({
+  syntax: "<transform-function># | none",
+  inherits: false,
+  initialValue: "none"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px), scale(2)", "translateX(200px), scale(4)"],
+  expected: "translateX(150px), scale(3)"
+}, 'Additive animation of <transform-function># when underlying value is none');
+
+test(() => {
+  const name = generate_name();
+  CSS.registerProperty({
+    name,
+    syntax: "<transform-function>#",
+    inherits: false,
+    initialValue: "translateX(10px), scale(2)"
+  });
+
+  target.style.setProperty(name, "translateX(10px)");
+  const anim = target.animate(
+    { [name]: ["translateX(20px), scale(3)", "translateX(40px), scale(5)"] },
+    { duration: 1000, composite: "add" }
+  );
+  anim.pause();
+  anim.currentTime = 500;
+
+  // Underlying list length (1) != keyframe list length (2), falls back to replace.
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(30px), scale(4)"
+  );
+
+  // Dynamically updating underlying list to length 2 invalidates cache and composites.
+  target.style.setProperty(name, "translateX(10px), scale(2)");
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(40px), scale(5)"
+  );
+
+  target.style.removeProperty(name);
+  anim.cancel();
+}, 'Additive <transform-function># animation with dynamic underlying list length and value update');
+
 </script>
diff --git a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
index 161703a6..f090251 100644
--- a/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
+++ b/third_party/blink/web_tests/external/wpt/css/css-properties-values-api/animation/custom-property-animation-transform-list-multiple-values.html
@@ -65,4 +65,123 @@
   assert_function: assert_matrix_equals
 }, 'Animating a custom property of type <transform-list> containing multiple values and with mismatching list lengths');
 
+animation_test({
+  syntax: "<transform-list> | none",
+  inherits: false,
+  initialValue: "none"
+}, {
+  composite: "add",
+  keyframes: ["translateX(100px) scale(2)", "translateX(200px) scale(4)"],
+  expected: "translateX(150px) scale(3)"
+}, 'Animating a custom property of type <transform-list> | none with additivity when underlying value is none');
+
+animation_test({
+  syntax: "<transform-list> | none",
+  inherits: false,
+  initialValue: "none"
+}, {
+  composite: "accumulate",
+  keyframes: ["translateX(100px) scale(2)", "translateX(200px) scale(4)"],
+  expected: "translateX(150px) scale(3)"
+}, 'Animating a custom property of type <transform-list> | none with accumulation when underlying value is none');
+
+test(() => {
+  const name = generate_name();
+  CSS.registerProperty({
+    name,
+    syntax: "<transform-list> | none",
+    inherits: false,
+    initialValue: "translateX(100px) scale(2)"
+  });
+
+  target.style.setProperty(name, "none");
+  const animation = target.animate(
+    { [name]: ["translateX(200px) scale(3)", "translateX(300px) scale(4)"] },
+    { duration: 1000, composite: "add" }
+  );
+  animation.pause();
+  animation.currentTime = 500;
+
+  // When the underlying value is none, composition falls back to replace.
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(250px) scale(3.5)"
+  );
+
+  // Dynamically restoring a valid underlying <transform-list> invalidates the
+  // conversion cache and composites with the new underlying value.
+  target.style.setProperty(name, "translateX(100px) scale(2)");
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(100px) scale(2) translateX(250px) scale(3.5)"
+  );
+
+  target.style.removeProperty(name);
+  animation.cancel();
+}, 'Additive <transform-list> | none animation with dynamic underlying value update from none');
+
+test(() => {
+  const name = generate_name();
+  CSS.registerProperty({
+    name,
+    syntax: "<transform-list>",
+    inherits: false,
+    initialValue: "translateX(100px) scale(2)"
+  });
+
+  const baseAnim = target.animate(
+    { [name]: ["var(--invalid, invalid)", "var(--invalid, invalid)"] },
+    { duration: 1000 }
+  );
+  baseAnim.pause();
+  baseAnim.currentTime = 500;
+
+  const addAnim = target.animate(
+    { [name]: ["translateX(200px) scale(3)", "translateX(300px) scale(4)"] },
+    { duration: 1000, composite: "add" }
+  );
+  addAnim.pause();
+  addAnim.currentTime = 500;
+
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(250px) scale(3.5)"
+  );
+
+  addAnim.cancel();
+  baseAnim.cancel();
+}, 'Additive <transform-list> animation stacked on top of a discrete var(--invalid, invalid) animation');
+
+test(() => {
+  const name = generate_name();
+  CSS.registerProperty({
+    name,
+    syntax: "<transform-list> | none",
+    inherits: false,
+    initialValue: "translateX(100px) scale(2)"
+  });
+
+  const baseAnim = target.animate(
+    { [name]: ["none", "none"] },
+    { duration: 1000 }
+  );
+  baseAnim.pause();
+  baseAnim.currentTime = 500;
+
+  const addAnim = target.animate(
+    { [name]: ["translateX(200px) scale(3)", "translateX(300px) scale(4)"] },
+    { duration: 1000, composite: "add" }
+  );
+  addAnim.pause();
+  addAnim.currentTime = 500;
+
+  assert_equals(
+    getComputedStyle(target).getPropertyValue(name),
+    "translateX(250px) scale(3.5)"
+  );
+
+  addAnim.cancel();
+  baseAnim.cancel();
+}, 'Additive <transform-list> animation stacked on top of a discrete none animation');
+
 </script>