diff --git a/dom/animation/Animation.cpp b/dom/animation/Animation.cpp
index ba06efccb904..a7077d712457 100644
--- a/dom/animation/Animation.cpp
+++ b/dom/animation/Animation.cpp
@@ -58,13 +58,7 @@ Animation::SetEffect(KeyframeEffectReadOnly* aEffect)
 void
 Animation::SetStartTime(const Nullable<TimeDuration>& aNewStartTime)
 {
-#if 1
-  // Bug 1096776: once we support inactive/missing timelines we'll want to take
-  // the disabled branch.
-  MOZ_ASSERT(mTimeline && !mTimeline->GetCurrentTime().IsNull(),
-             "We don't support inactive/missing timelines yet");
-#else
-  Nullable<TimeDuration> timelineTime = mTimeline->GetCurrentTime();
+  Nullable<TimeDuration> timelineTime;
   if (mTimeline) {
     // The spec says to check if the timeline is active (has a resolved time)
     // before using it here, but we don't need to since it's harmless to set
@@ -74,7 +68,7 @@ Animation::SetStartTime(const Nullable<TimeDuration>& aNewStartTime)
   if (timelineTime.IsNull() && !aNewStartTime.IsNull()) {
     mHoldTime.SetNull();
   }
-#endif
+
   Nullable<TimeDuration> previousCurrentTime = GetCurrentTime();
   mStartTime = aNewStartTime;
   if (!aNewStartTime.IsNull()) {
@@ -106,7 +100,7 @@ Animation::GetCurrentTime() const
     return result;
   }
 
-  if (!mStartTime.IsNull()) {
+  if (mTimeline && !mStartTime.IsNull()) {
     Nullable<TimeDuration> timelineTime = mTimeline->GetCurrentTime();
     if (!timelineTime.IsNull()) {
       result.SetValue((timelineTime.Value() - mStartTime.Value())
@@ -315,6 +309,8 @@ Animation::Tick()
   // resuming.
   if (mPendingState != PendingState::NotPending &&
       !mPendingReadyTime.IsNull() &&
+      mTimeline &&
+      !mTimeline->GetCurrentTime().IsNull() &&
       mPendingReadyTime.Value() <= mTimeline->GetCurrentTime().Value()) {
     FinishPendingAt(mPendingReadyTime.Value());
     mPendingReadyTime.SetNull();
@@ -355,8 +351,15 @@ Animation::TriggerNow()
   if (PlayState() != AnimationPlayState::Pending) {
     return;
   }
-  MOZ_ASSERT(mTimeline && !mTimeline->GetCurrentTime().IsNull(),
-             "Expected an active timeline");
+
+  // If we don't have an active timeline we can't trigger the animation.
+  // However, this is a test-only method that we don't expect to be used in
+  // conjunction with animations without an active timeline so generate
+  // a warning if we do find ourselves in that situation.
+  if (!mTimeline || mTimeline->GetCurrentTime().IsNull()) {
+    NS_WARNING("Failed to trigger an animation with an active timeline");
+    return;
+  }
 
   FinishPendingAt(mTimeline->GetCurrentTime().Value());
 }
@@ -797,10 +800,12 @@ Animation::UpdateFinishedState(SeekFlag aSeekFlag)
         mHoldTime.SetValue(0);
       }
     } else if (mPlaybackRate != 0.0 &&
-               !currentTime.IsNull()) {
+               !currentTime.IsNull() &&
+               mTimeline &&
+               !mTimeline->GetCurrentTime().IsNull()) {
       if (aSeekFlag == SeekFlag::DidSeek && !mHoldTime.IsNull()) {
         mStartTime.SetValue(mTimeline->GetCurrentTime().Value() -
-                              (mHoldTime.Value().MultDouble(1 / mPlaybackRate)));
+                             (mHoldTime.Value().MultDouble(1 / mPlaybackRate)));
       }
       mHoldTime.SetNull();
     }
diff --git a/dom/animation/PendingAnimationTracker.cpp b/dom/animation/PendingAnimationTracker.cpp
index 99191bca958b..46ab692eccbb 100644
--- a/dom/animation/PendingAnimationTracker.cpp
+++ b/dom/animation/PendingAnimationTracker.cpp
@@ -55,6 +55,13 @@ TriggerAnimationAtTime(nsRefPtrHashKey<dom::Animation>* aKey,
   dom::Animation* animation = aKey->GetKey();
   dom::AnimationTimeline* timeline = animation->Timeline();
 
+  // If the animation does not have a timeline, just drop it from the map.
+  // The animation will detect that it is not being tracked and will trigger
+  // itself on the next tick where it has a timeline.
+  if (!timeline) {
+    return PL_DHASH_REMOVE;
+  }
+
   // When the timeline's refresh driver is under test control, its values
   // have no correspondance to wallclock times so we shouldn't try to convert
   // aReadyTime (which is a wallclock time) to a timeline value. Instead, the
diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index 5a7f0d0185ca..fd449990d57c 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -364,6 +364,11 @@ AddAnimationForProperty(nsIFrame* aFrame, const AnimationProperty& aProperty,
   MOZ_ASSERT(aLayer->AsContainerLayer(), "Should only animate ContainerLayer");
   MOZ_ASSERT(aAnimation->GetEffect(),
              "Should not be adding an animation without an effect");
+  MOZ_ASSERT(!aAnimation->GetCurrentOrPendingStartTime().IsNull() ||
+             (aAnimation->Timeline() &&
+              aAnimation->Timeline()->TracksWallclockTime()),
+             "Animation should either have a resolved start time or "
+             "a timeline that tracks wallclock time");
   nsStyleContext* styleContext = aFrame->StyleContext();
   nsPresContext* presContext = aFrame->PresContext();
   TransformReferenceBox refBox(aFrame);
@@ -462,7 +467,8 @@ AddAnimationsForProperty(nsIFrame* aFrame, nsCSSProperty aProperty,
     // driver under test control. In this case, the next time the refresh
     // driver is advanced it will trigger any pending animations.
     if (anim->PlayState() == AnimationPlayState::Pending &&
-        !anim->Timeline()->TracksWallclockTime()) {
+        (!anim->Timeline() ||
+         !anim->Timeline()->TracksWallclockTime())) {
       continue;
     }