diff --git a/dom/html/HTMLMediaElement.cpp b/dom/html/HTMLMediaElement.cpp
index 579119163076..fdf0c9733518 100644
--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -4513,6 +4513,15 @@ void HTMLMediaElement::UnbindFromTree(bool aDeep,
   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,
@@ -4522,7 +4531,16 @@ HTMLMediaElement::GetCanPlay(const nsAString& 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
diff --git a/dom/media/test/test_can_play_type_mpeg.html b/dom/media/test/test_can_play_type_mpeg.html
index 43ff2c1a8b1b..744d2237d397 100644
--- a/dom/media/test/test_can_play_type_mpeg.html
+++ b/dom/media/test/test_can_play_type_mpeg.html
@@ -88,6 +88,23 @@ function check_mp4(v, enabled) {
   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) {