--- a/dom/animation/KeyframeUtils.cpp
+++ b/dom/animation/KeyframeUtils.cpp
@@ -1482,17 +1482,19 @@ GetKeyframeListFromPropertyIndexedKeyfra
       aRv.Throw(NS_ERROR_DOM_ANIM_MISSING_PROPS_ERR);
       return;
     }
 
     size_t n = pair.mValues.Length() - 1;
     size_t i = 0;
 
     for (const nsString& stringValue : pair.mValues) {
-      double offset = i++ / double(n);
+      // For single-valued lists, the single value should be added to a
+      // keyframe with offset 1.
+      double offset = n ? i++ / double(n) : 1;
       Keyframe* keyframe = processedKeyframes.LookupOrAdd(offset);
       if (keyframe->mPropertyValues.IsEmpty()) {
         keyframe->mTimingFunction = easing;
         keyframe->mComposite = composite;
         keyframe->mComputedOffset = offset;
       }
       keyframe->mPropertyValues.AppendElement(
         MakePropertyValuePair(pair.mProperty, stringValue, parser, aDocument));

--- a/testing/web-platform/meta/web-animations/interfaces/KeyframeEffect/processing-a-keyframes-argument.html.ini
+++ b/testing/web-platform/meta/web-animations/interfaces/KeyframeEffect/processing-a-keyframes-argument.html.ini
@@ -1,8 +1,5 @@
 [processing-a-keyframes-argument.html]
   type: testharness
-  [Equivalent property indexed and sequenced keyframes: two properties with one value]
-    expected: FAIL
-
   [Equivalent property indexed and sequenced keyframes: same offset applied to all keyframes]
     expected: FAIL