--- a/dom/media/platforms/agnostic/AOMDecoder.cpp
+++ b/dom/media/platforms/agnostic/AOMDecoder.cpp
@@ -216,16 +216,29 @@ bool
 AOMDecoder::IsAV1(const nsACString& aMimeType)
 {
   return aMimeType.EqualsLiteral("video/webm; codecs=av1")
          || aMimeType.EqualsLiteral("video/av1");
 }
 
 /* static */
 bool
+AOMDecoder::IsSupportedCodec(const nsAString& aCodecType)
+{
+  // While AV1 is under development, we describe support
+  // for a specific aom commit hash so sites can check
+  // compatibility.
+  auto version = NS_ConvertASCIItoUTF16("av1.experimental.");
+  version.AppendLiteral("4d668d7feb1f8abd809d1bca0418570a7f142a36");
+  return aCodecType.EqualsLiteral("av1") ||
+         aCodecType.Equals(version);
+}
+
+/* static */
+bool
 AOMDecoder::IsKeyframe(Span<const uint8_t> aBuffer) {
   aom_codec_stream_info_t info;
   PodZero(&info);
   info.sz = sizeof(info);
 
   auto res = aom_codec_peek_stream_info(aom_codec_av1_dx(),
                                         aBuffer.Elements(),
                                         aBuffer.Length(),

--- a/dom/media/platforms/agnostic/AOMDecoder.h
+++ b/dom/media/platforms/agnostic/AOMDecoder.h
@@ -28,16 +28,19 @@ public:
   {
     return "libaom (AV1) video decoder";
   }
 
   // Return true if aMimeType is a one of the strings used
   // by our demuxers to identify AV1 streams.
   static bool IsAV1(const nsACString& aMimeType);
 
+  // Return true if aCodecType is a supported codec description.
+  static bool IsSupportedCodec(const nsAString& aCodecType);
+
   // Return true if a sample is a keyframe.
   static bool IsKeyframe(Span<const uint8_t> aBuffer);
 
   // Return the frame dimensions for a sample.
   static nsIntSize GetFrameSize(Span<const uint8_t> aBuffer);
 
 private:
   ~AOMDecoder();

--- a/dom/media/webm/WebMDecoder.cpp
+++ b/dom/media/webm/WebMDecoder.cpp
@@ -1,15 +1,18 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/Preferences.h"
+#ifdef MOZ_AV1
+#include "AOMDecoder.h"
+#endif
 #include "MediaContainerType.h"
 #include "MediaDecoderStateMachine.h"
 #include "WebMDemuxer.h"
 #include "WebMDecoder.h"
 #include "VideoUtils.h"
 
 namespace mozilla {
 
@@ -46,16 +49,21 @@ WebMDecoder::IsSupportedType(const Media
     }
     // Note: Only accept VP8/VP9 in a video container type, not in an audio
     // container type.
     if (isVideo &&
         (codec.EqualsLiteral("vp8") || codec.EqualsLiteral("vp8.0") ||
          codec.EqualsLiteral("vp9") || codec.EqualsLiteral("vp9.0"))) {
       continue;
     }
+#ifdef MOZ_AV1
+    if (isVideo && AOMDecoder::IsSupportedCodec(codec)) {
+      continue;
+    }
+#endif
     // Some unsupported codec.
     return false;
   }
   return true;
 }
 
 void
 WebMDecoder::GetMozDebugReaderData(nsACString& aString)