--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -4508,26 +4508,44 @@ void HTMLMediaElement::UnbindFromTree(bo
   nsCOMPtr<nsIRunnable> task = NS_NewRunnableFunction([self] () {
     if (self->mUnboundFromTree) {
       self->Pause();
     }
   });
   RunInStableState(task);
 }
 
+static bool
+IsVP9InMP4(const MediaContainerType& aContainerType)
+{
+  const MediaContainerType mimeType(aContainerType.Type());
+  return DecoderTraits::IsMP4SupportedType(mimeType,
+                                           /* DecoderDoctorDiagnostics* */ nullptr)
+         && IsVP9CodecString(aContainerType.ExtendedType().Codecs().AsString());
+}
+
 /* static */
 CanPlayStatus
 HTMLMediaElement::GetCanPlay(const nsAString& aType,
                              DecoderDoctorDiagnostics* aDiagnostics)
 {
   Maybe<MediaContainerType> containerType = MakeMediaContainerType(aType);
   if (!containerType) {
     return CANPLAY_NO;
   }
-  return DecoderTraits::CanHandleContainerType(*containerType, aDiagnostics);
+  CanPlayStatus status = DecoderTraits::CanHandleContainerType(*containerType, aDiagnostics);
+  if (status == CANPLAY_YES && IsVP9InMP4(*containerType)) {
+    // We don't have a demuxer that can handle VP9 in non-fragmented MP4.
+    // So special-case VP9 in MP4 here, as we assume canPlayType() implies
+    // non-fragmented MP4 anyway. Note we report that we can play VP9
+    // in MP4 in MediaSource.isTypeSupported(), as the fragmented MP4
+    // demuxer can handle VP9 in fragmented MP4.
+    return CANPLAY_NO;
+  }
+  return status;
 }
 
 NS_IMETHODIMP
 HTMLMediaElement::CanPlayType(const nsAString& aType, nsAString& aResult)
 {
   DecoderDoctorDiagnostics diagnostics;
   CanPlayStatus canPlay = GetCanPlay(aType, &diagnostics);
   diagnostics.StoreFormatDiagnostics(

--- a/dom/media/test/test_can_play_type_mpeg.html
+++ b/dom/media/test/test_can_play_type_mpeg.html
@@ -83,16 +83,33 @@ function check_mp4(v, enabled) {
   check("audio/mp4; codecs=\"opus\"", "probably");
   check("audio/mp4; codecs=opus", "probably");
 
   // Flac.
   // Not available on Android yet.
   var expectedResult = IsSupportedAndroid() ? "" : "probably";
   check("audio/mp4; codecs=\"flac\"", expectedResult);
   check("audio/mp4; codecs=flac", expectedResult);
+
+  // VP9.
+  // Note: canPlayType assumes non-fragmented MP4. Once we support VP9 in
+  // non-fragmented MP4, we need to change this to report supported.
+  [ "video/mp4; codecs=vp9",
+    "video/mp4; codecs=\"vp9\"",
+    "video/mp4; codecs=\"vp9.0\""
+  ].forEach((codec) => {
+    // canPlayType should not support VP9 in MP4...
+    check(codec, "");
+    // But VP9 in MP4 is supported in MSE.
+    ok(MediaSource.isTypeSupported(codec), "VP9 in MP4 should be supported in MSE");
+  });
+
+  check("video/mp4; codecs=vp9", "");
+  check("video/mp4; codecs=\"vp9\"", "");
+  check("video/mp4; codecs=\"vp9.0\"", "");
 }
 
 function check_mp3(v, enabled) {
   function check(type, expected) {
     var ex = enabled ? expected : "";
     is(v.canPlayType(type), ex, type + "='" + ex + "'");
   }