--- a/dom/animation/EffectCompositor.cpp
+++ b/dom/animation/EffectCompositor.cpp
@@ -995,16 +995,27 @@ EffectCompositor::PreTraverseInSubtree(E
     // (skippable) restyle, so we can skip it if flushThrottledRestyles is not
     // true.
     if (!flushThrottledRestyles && !aIter.Data()) {
       return returnTarget;
     }
 
     const NonOwningAnimationTarget& target = aIter.Key();
 
+    // Skip elements in documents without a pres shell. Normally we filter out
+    // such elements in RequestRestyle but it can happen that, after adding
+    // them to mElementsToRestyle, they are transferred to a different document.
+    //
+    // We will drop them from mElementsToRestyle at the end of the next full
+    // document restyle (at the end of this function) but for consistency with
+    // how we treat such elements in RequestRestyle, we just ignore them here.
+    if (!nsComputedDOMStyle::GetPresShellForContent(target.mElement)) {
+      return returnTarget;
+    }
+
     // Ignore restyles that aren't in the flattened tree subtree rooted at
     // aRoot.
     if (aRoot &&
         !nsContentUtils::ContentIsFlattenedTreeDescendantOf(target.mElement,
                                                             aRoot)) {
       return returnTarget;
     }
 
@@ -1075,17 +1086,26 @@ EffectCompositor::PreTraverseInSubtree(E
       for (KeyframeEffectReadOnly* effect : *effects) {
         effect->GetAnimation()->WillComposeStyle();
       }
 
       // Remove the element from the list of elements to restyle since we are
       // about to restyle it.
       iter.Remove();
     }
+
+    // If this is a full document restyle, then unconditionally clear
+    // elementSet in case there are any elements that didn't match above
+    // because they were moved to a document without a pres shell after
+    // posting an animation restyle.
+    if (!aRoot && flushThrottledRestyles) {
+      elementSet.Clear();
+    }
   }
+
   return foundElementsNeedingRestyle;
 }
 
 bool
 EffectCompositor::PreTraverse(dom::Element* aElement,
                               CSSPseudoElementType aPseudoType)
 {
   MOZ_ASSERT(NS_IsMainThread());

--- a/dom/animation/test/mozilla/file_restyling_xhr_doc.html
+++ b/dom/animation/test/mozilla/file_restyling_xhr_doc.html
@@ -76,11 +76,47 @@ promise_test(t => {
     // Gecko currently skips applying animation styles to elements in documents
     // without browsing contexts.
     assert_not_equals(getComputedStyle(div).opacity, '0',
                       'Style should NOT be updated');
   });
 }, 'Forcing an animation targetting an element in a document without a'
     + ' browsing context to play does not cause style to update');
 
+// Following is an additional Gecko-specific test to confirm the behavior
+// when we have an element with an animation restyle and then move it to
+// a document without a pres shell.
+
+promise_test(t => {
+  let anim;
+  return getXHRDoc(t).then(xhrdoc => {
+    const div = addDiv(t);
+    anim = div.animate({ opacity: [ 0, 1 ] }, 1000);
+    assert_equals(getComputedStyle(div).opacity, '0',
+                  'Style should be updated');
+    // Trigger an animation restyle to be queued
+    anim.currentTime = 0.1;
+    // Adopt node into XHR doc
+    xhrdoc.body.appendChild(div);
+    // We should skip applying animation styles to elements in documents
+    // without a pres shell.
+    //
+    // The Gecko style backend, however, does not do this. Since we expect the
+    // Gecko style backend to be obsolete in the near future, we only perform
+    // this check when the Servo backend is in use.
+    let isServo = false;
+    try {
+      isServo = SpecialPowers.getBoolPref('layout.css.servo.enabled');
+    } catch(e) {
+      // getBoolPref throws if the pref does not exist, in which case isServo
+      // should remain false.
+    }
+    if (isServo) {
+      assert_equals(getComputedStyle(div).opacity, '1',
+                    'Style should NOT be updated');
+    }
+  });
+}, 'Moving an element with a pending animation restyle to a document without'
+   + ' a browsing context resets animation style');
+
 done();
 
 </script>