--- a/testing/web-platform/meta/web-animations/interfaces/Animatable/animate.html.ini
+++ b/testing/web-platform/meta/web-animations/interfaces/Animatable/animate.html.ini
@@ -21,14 +21,20 @@
   [Element.animate() accepts a property-indexed keyframe with an array of easings]
     expected: FAIL
   [Element.animate() accepts a property-indexed keyframe with an array of easings that is too short]
     expected: FAIL
   [Element.animate() accepts a property-indexed keyframe with an array of easings that is too long]
     expected: FAIL
   [Element.animate() accepts a property-indexed keyframe with an empty array of easings]
     expected: FAIL
+  [Element.animate() accepts a property-indexed keyframe with a composite array]
+    expected: FAIL
+  [Element.animate() accepts a property-indexed keyframe with a composite array that is too short]
+    expected: FAIL
+  [Element.animate() accepts a property-indexed keyframe with a composite array that is too long]
+    expected: FAIL
   [Element.animate() does not accept property-indexed keyframes not loosely sorted by offset]
     expected: FAIL
   [Element.animate() does not accept property-indexed keyframes not loosely sorted by offset even though not all offsets are specified]
     expected: FAIL
   [Element.animate() does not accept property-indexed keyframes with offsets out of range]
     expected: FAIL

--- a/testing/web-platform/meta/web-animations/interfaces/KeyframeEffect/constructor.html.ini
+++ b/testing/web-platform/meta/web-animations/interfaces/KeyframeEffect/constructor.html.ini
@@ -29,14 +29,26 @@
   [a KeyframeEffectReadOnly can be constructed with a property-indexed keyframe with an array of easings that is too long]
     expected: FAIL
   [a KeyframeEffectReadOnly constructed with a property-indexed keyframe with an array of easings that is too long roundtrips]
     expected: FAIL
   [a KeyframeEffectReadOnly can be constructed with a property-indexed keyframe with an empty array of easings]
     expected: FAIL
   [a KeyframeEffectReadOnly constructed with a property-indexed keyframe with an empty array of easings roundtrips]
     expected: FAIL
+  [a KeyframeEffectReadOnly can be constructed with a property-indexed keyframe with a composite array]
+    expected: FAIL
+  [a KeyframeEffectReadOnly constructed with a property-indexed keyframe with a composite array roundtrips]
+    expected: FAIL
+  [a KeyframeEffectReadOnly can be constructed with a property-indexed keyframe with a composite array that is too short]
+    expected: FAIL
+  [a KeyframeEffectReadOnly constructed with a property-indexed keyframe with a composite array that is too short roundtrips]
+    expected: FAIL
+  [a KeyframeEffectReadOnly can be constructed with a property-indexed keyframe with a composite array that is too long]
+    expected: FAIL
+  [a KeyframeEffectReadOnly constructed with a property-indexed keyframe with a composite array that is too long roundtrips]
+    expected: FAIL
   [KeyframeEffectReadOnly constructor throws with property-indexed keyframes not loosely sorted by offset]
     expected: FAIL
   [KeyframeEffectReadOnly constructor throws with property-indexed keyframes not loosely sorted by offset even though not all offsets are specified]
     expected: FAIL
   [KeyframeEffectReadOnly constructor throws with property-indexed keyframes with offsets out of range]
     expected: FAIL

--- a/testing/web-platform/meta/web-animations/interfaces/KeyframeEffect/setKeyframes.html.ini
+++ b/testing/web-platform/meta/web-animations/interfaces/KeyframeEffect/setKeyframes.html.ini
@@ -21,14 +21,20 @@
   [Keyframes can be replaced with a property-indexed keyframe with an array of easings]
     expected: FAIL
   [Keyframes can be replaced with a property-indexed keyframe with an array of easings that is too short]
     expected: FAIL
   [Keyframes can be replaced with a property-indexed keyframe with an array of easings that is too long]
     expected: FAIL
   [Keyframes can be replaced with a property-indexed keyframe with an empty array of easings]
     expected: FAIL
+  [Keyframes can be replaced with a property-indexed keyframe with a composite array]
+    expected: FAIL
+  [Keyframes can be replaced with a property-indexed keyframe with a composite array that is too short]
+    expected: FAIL
+  [Keyframes can be replaced with a property-indexed keyframe with a composite array that is too long]
+    expected: FAIL
   [KeyframeEffect constructor throws with property-indexed keyframes not loosely sorted by offset]
     expected: FAIL
   [KeyframeEffect constructor throws with property-indexed keyframes not loosely sorted by offset even though not all offsets are specified]
     expected: FAIL
   [KeyframeEffect constructor throws with property-indexed keyframes with offsets out of range]
     expected: FAIL

--- a/testing/web-platform/tests/web-animations/resources/keyframe-tests.js
+++ b/testing/web-platform/tests/web-animations/resources/keyframe-tests.js
@@ -313,16 +313,71 @@ const gKeyframesTests = [
             + ' long',
     input:  { left: ['10px', '20px', '30px'],
               easing: ['steps(1)', 'steps(2)', 'steps(3)', 'steps(4)'] },
     output: [keyframe(computedOffset(0),   { left: '10px' }, 'steps(1)'),
              keyframe(computedOffset(0.5), { left: '20px' }, 'steps(2)'),
              keyframe(computedOffset(1),   { left: '30px' }, 'steps(3)')],
   },
 
+  // ----------- Property-indexed keyframes: composite handling -----------
+
+  {
+    desc:   'a property-indexed keyframe with a single composite operation',
+    input:  { left: ['10px', '20px', '30px'], composite: 'add' },
+    output: [keyframe(computedOffset(0),   { left: '10px' }, 'linear', 'add'),
+             keyframe(computedOffset(0.5), { left: '20px' }, 'linear', 'add'),
+             keyframe(computedOffset(1),   { left: '30px' }, 'linear', 'add')],
+  },
+  {
+    desc:   'a property-indexed keyframe with a composite array',
+    input:  { left: ['10px', '20px', '30px'],
+              composite: ['add', 'replace', 'accumulate'] },
+    output: [keyframe(computedOffset(0),   { left: '10px' },
+                      'linear', 'add'),
+             keyframe(computedOffset(0.5), { left: '20px' },
+                      'linear', 'replace'),
+             keyframe(computedOffset(1),   { left: '30px' },
+                      'linear', 'accumulate')],
+  },
+  {
+    desc:   'a property-indexed keyframe with a composite array that is too'
+            + ' short',
+    input:  { left: ['10px', '20px', '30px', '40px', '50px'],
+              composite: ['add', 'replace'] },
+    output: [keyframe(computedOffset(0),    { left: '10px' },
+                      'linear', 'add'),
+             keyframe(computedOffset(0.25), { left: '20px' },
+                      'linear', 'replace'),
+             keyframe(computedOffset(0.5),  { left: '30px' },
+                      'linear', 'add'),
+             keyframe(computedOffset(0.75), { left: '40px' },
+                      'linear', 'replace'),
+             keyframe(computedOffset(1),    { left: '50px' },
+                      'linear', 'add')],
+  },
+  {
+    desc:   'a property-indexed keyframe with a composite array that is too'
+            + ' long',
+    input:  { left: ['10px', '20px'],
+              composite: ['add', 'replace', 'accumulate'] },
+    output: [keyframe(computedOffset(0), { left: '10px' },
+                      'linear', 'add'),
+             keyframe(computedOffset(1), { left: '20px' },
+                      'linear', 'replace')],
+  },
+  {
+    desc:   'a property-indexed keyframe with a single-element composite array',
+    input:  { left: ['10px', '20px', '30px'],
+              composite: ['add'] },
+    output: [keyframe(computedOffset(0),   { left: '10px' }, 'linear', 'add'),
+             keyframe(computedOffset(0.5), { left: '20px' }, 'linear', 'add'),
+             keyframe(computedOffset(1),   { left: '30px' }, 'linear', 'add')],
+  },
+
   // ----------- Keyframe sequence: property handling -----------
 
   {
     desc:   'a one property one keyframe sequence',
     input:  [{ offset: 1, left: '10px' }],
     output: [keyframe(offset(1), { left: '10px' })],
   },
   {
@@ -603,16 +658,27 @@ const gInvalidKeyframesTests = [
     input:  { easing: ['invalid'] },
   },
   {
     desc:  'a keyframe sequence with an invalid easing value',
     input: [ { opacity: 0, easing: 'jumpy' },
              { opacity: 1 } ],
   },
   {
+    desc:  'property-indexed keyframes with an invalid composite value',
+    input: { opacity: [ 0, 0.5, 1 ],
+             composite: 'alternate' },
+  },
+  {
+    desc:  'property-indexed keyframes with an invalid composite value as one'
+           + ' of the array values',
+    input: { opacity: [ 0, 0.5, 1 ],
+             composite: [ 'add', 'alternate' ] },
+  },
+  {
     desc:  'keyframes with an invalid composite value',
     input: [ { opacity: 0, composite: 'alternate' },
              { opacity: 1 } ],
   },
 ];
 
 // ------------------------------
 //  KeyframeEffectOptions