--- a/dom/animation/AnimationEffectReadOnly.cpp
+++ b/dom/animation/AnimationEffectReadOnly.cpp
@@ -195,30 +195,28 @@ AnimationEffectReadOnly::GetComputedTimi
 
   // Convert the overall progress to a fraction of a single iteration--the
   // simply iteration progress.
   // https://w3c.github.io/web-animations/#simple-iteration-progress
   double progress = IsFinite(overallProgress)
                     ? fmod(overallProgress, 1.0)
                     : fmod(result.mIterationStart, 1.0);
 
-  // When we finish exactly at the end of an iteration we need to report
-  // the end of the final iteration and not the start of the next iteration.
-  // We *don't* want to do this when we have a zero-iteration animation or
-  // when the animation has been effectively made into a zero-duration animation
-  // using a negative end-delay, however.
-  if (result.mPhase == ComputedTiming::AnimationPhase::After &&
-      progress == 0.0 &&
-      result.mIterations != 0.0 &&
-      (result.mActiveTime != zeroDuration ||
-       result.mDuration == zeroDuration)) {
-    // The only way we can be in the after phase with a progress of zero and
-    // a current iteration of zero, is if we have a zero iteration count or
-    // were clipped using a negative end delay--both of which we should have
-    // detected above.
+  // When we are at the end of the active interval and the end of an iteration
+  // we need to report the end of the final iteration and not the start of the
+  // next iteration. We *don't* want to do this, however, when we have
+  // a zero-iteration animation.
+  if (progress == 0.0 &&
+      (result.mPhase == ComputedTiming::AnimationPhase::After ||
+       result.mPhase == ComputedTiming::AnimationPhase::Active) &&
+      result.mActiveTime == result.mActiveDuration &&
+      result.mIterations != 0.0) {
+    // The only way we can reach the end of the active interval and have
+    // a progress of zero and a current iteration of zero, is if we have a zero
+    // iteration count -- something we should have detected above.
     MOZ_ASSERT(result.mCurrentIteration != 0,
                "Should not have zero current iteration");
     progress = 1.0;
     if (result.mCurrentIteration != UINT64_MAX) {
       result.mCurrentIteration--;
     }
   }
 

deleted file mode 100644
--- a/testing/web-platform/meta/web-animations/timing-model/animation-effects/current-iteration.html.ini
+++ /dev/null
@@ -1,7 +0,0 @@
-[current-iteration.html]
-  type: testharness
-  [Test negative playback rate: duration:1 delay:1 fill:both playbackRate:-1]
-    expected: FAIL
-
-  [Test negative playback rate: duration:1 delay:1 iterations:2 fill:both playbackRate:-1]
-    expected: FAIL

deleted file mode 100644
--- a/testing/web-platform/meta/web-animations/timing-model/animation-effects/simple-iteration-progress.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[simple-iteration-progress.html]
-  type: testharness
-  [Test negative playback rate: duration:1 delay:1 fill:both playbackRate:-1]
-    expected: FAIL

--- a/testing/web-platform/tests/web-animations/timing-model/animation-effects/current-iteration.html
+++ b/testing/web-platform/tests/web-animations/timing-model/animation-effects/current-iteration.html
@@ -512,17 +512,17 @@ runTests([
   {
     input:    { iterationStart: 0.5,
                 duration: 100,
                 delay: 1,
                 fill: 'both',
                 endDelay: -50 },
     before: 0,
     active: 0,
-    after: 0
+    after: 1
   },
 
   {
     input:    { iterationStart: 0.5,
                 duration: 100,
                 delay: 1,
                 fill: 'both',
                 endDelay: -100 },
@@ -534,17 +534,17 @@ runTests([
   {
     input:    { iterations: 2,
                 duration: 100,
                 delay: 1,
                 fill: 'both',
                 endDelay: -100 },
     before: 0,
     active: 0,
-    after: 0
+    after: 1
   },
 
   {
     input:    { iterations: 1,
                 iterationStart: 2,
                 duration: 100,
                 delay: 1,
                 fill: 'both',

--- a/testing/web-platform/tests/web-animations/timing-model/animation-effects/simple-iteration-progress.html
+++ b/testing/web-platform/tests/web-animations/timing-model/animation-effects/simple-iteration-progress.html
@@ -503,17 +503,17 @@ runTests([
   {
     input:    { iterationStart: 0.5,
                 duration: 100,
                 delay: 1,
                 fill: 'both',
                 endDelay: -50 },
     before: 0.5,
     active: 0.5,
-    after: 1
+    after: 0
   },
 
   {
     input:    { iterationStart: 0.5,
                 duration: 100,
                 delay: 1,
                 fill: 'both',
                 endDelay: -100 },
@@ -525,17 +525,17 @@ runTests([
   {
     input:    { iterations: 2,
                 duration: 100,
                 delay: 1,
                 fill: 'both',
                 endDelay: -100 },
     before: 0,
     active: 0,
-    after: 1
+    after: 0
   },
 
   {
     input:    { iterations: 1,
                 iterationStart: 2,
                 duration: 100,
                 delay: 1,
                 fill: 'both',