--- a/dom/animation/KeyframeEffect.cpp
+++ b/dom/animation/KeyframeEffect.cpp
@@ -266,17 +266,19 @@ KeyframeEffectReadOnly::GetComputedTimin
   if (localTime >=
         std::min(StickyTimeDuration(aTiming.mDelay + result.mActiveDuration),
                  result.mEndTime)) {
     result.mPhase = ComputedTiming::AnimationPhase::After;
     if (!result.FillsForwards()) {
       // The animation isn't active or filling at this time.
       return result;
     }
-    activeTime = result.mActiveDuration;
+    activeTime = std::max(std::min(result.mActiveDuration,
+                                   result.mActiveDuration + aTiming.mEndDelay),
+                          zeroDuration);
   } else if (localTime <
                std::min(StickyTimeDuration(aTiming.mDelay), result.mEndTime)) {
     result.mPhase = ComputedTiming::AnimationPhase::Before;
     if (!result.FillsBackwards()) {
       // The animation isn't active or filling at this time.
       return result;
     }
     // activeTime is zero
@@ -316,19 +318,29 @@ KeyframeEffectReadOnly::GetComputedTimin
   // 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.mIterations != 0.0 &&
+      (activeTime != 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.
+    MOZ_ASSERT(result.mCurrentIteration != 0,
+               "Should not have zero current iteration");
     progress = 1.0;
     if (result.mCurrentIteration != UINT64_MAX) {
       result.mCurrentIteration--;
     }
   }
 
   // Factor in the direction.
   bool thisIterationReverse = false;

--- a/layout/style/test/file_animations_effect_timing_enddelay.html
+++ b/layout/style/test/file_animations_effect_timing_enddelay.html
@@ -117,29 +117,29 @@ addAsyncAnimTest(function *() {
   advance_clock(400);
   yield waitForPaints();
   omta_is(div, "transform", { tx: 40 }, RunningOn.Compositor,
           "Animation is updated on compositor " +
           "duration 1000, endDelay -500, fill forwards, current time 400");
 
   advance_clock(100);
   yield waitForPaints();
-  omta_todo_is(div, "transform", { tx: 50 }, RunningOn.MainThread,
+  omta_is(div, "transform", { tx: 50 }, RunningOn.MainThread,
           "Animation is updated on main thread " +
           "duration 1000, endDelay -500, fill forwards, current time 500");
 
   advance_clock(400);
   yield waitForPaints();
-  omta_todo_is(div, "transform", { tx: 50 }, RunningOn.MainThread,
+  omta_is(div, "transform", { tx: 50 }, RunningOn.MainThread,
           "Animation is updated on main thread " +
           "duration 1000, endDelay -500, fill forwards, current time 900");
 
   advance_clock(100);
   yield waitForPaints();
-  omta_todo_is(div, "transform", { tx: 50 }, RunningOn.MainThread,
+  omta_is(div, "transform", { tx: 50 }, RunningOn.MainThread,
           "Animation is updated on main thread " +
           "duration 1000, endDelay -500, fill forwards, current time 1000");
 
   done_div();
 });
 
 </script>
 </body>

deleted file mode 100644
--- a/testing/web-platform/meta/web-animations/interfaces/AnimationEffectTiming/getComputedStyle.html.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[getComputedStyle.html]
-  type: testharness
-  [change currentTime when fill forwards and endDelay is negative]
-    expected: FAIL
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1283387

deleted file mode 100644
--- a/testing/web-platform/meta/web-animations/timing-model/animation-effects/active-time.html.ini
+++ /dev/null
@@ -1,13 +0,0 @@
-[active-time.html]
-  type: testharness
-  [Active time in after phase with forwards fill and negative end delay is the active duration + end delay]
-    expected: FAIL
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1283387
-
-  [Active time in after phase with forwards fill and negative end delay greater in magnitude than the active duration is zero]
-    expected: FAIL
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1283387
-
-  [Active time in after phase with forwards fill and negative end delay greater in magnitude than the sum of the active duration and start delay is zero]
-    expected: FAIL
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1283387