diff --git a/dom/html/HTMLMediaElement.cpp b/dom/html/HTMLMediaElement.cpp
index 675537625b31..7ad1d63c2121 100644
--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -2803,6 +2803,8 @@ nsresult HTMLMediaElement::InitializeDecoderAsClone(MediaDecoder* aOriginal)
   LOG(LogLevel::Debug, ("%p Cloned decoder %p from %p", this, decoder.get(), aOriginal));
 
   decoder->SetMediaSeekable(aOriginal->IsMediaSeekable());
+  decoder->SetMediaSeekableOnlyInBufferedRanges(
+    aOriginal->IsMediaSeekableOnlyInBufferedRanges());
 
   RefPtr<MediaResource> resource =
     originalResource->CloneData(decoder->GetResourceCallback());
diff --git a/dom/media/MediaDataDemuxer.h b/dom/media/MediaDataDemuxer.h
index 57ee35dbb83c..59c72689961e 100644
--- a/dom/media/MediaDataDemuxer.h
+++ b/dom/media/MediaDataDemuxer.h
@@ -70,6 +70,10 @@ public:
   // Returns true if the underlying resource allows seeking.
   virtual bool IsSeekable() const = 0;
 
+  // Returns true if the underlying resource can only seek within buffered
+  // ranges.
+  virtual bool IsSeekableOnlyInBufferedRanges() const { return false; }
+
   // Returns the media's crypto information, or nullptr if media isn't
   // encrypted.
   virtual UniquePtr<EncryptionInfo> GetCrypto()
diff --git a/dom/media/MediaDecoder.cpp b/dom/media/MediaDecoder.cpp
index 7f7e6198bc7f..accdf4c8d50a 100644
--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -564,6 +564,8 @@ MediaDecoder::MediaDecoder(MediaDecoderOwner* aOwner)
                      "MediaDecoder::mDecoderPosition (Canonical)")
   , mMediaSeekable(AbstractThread::MainThread(), true,
                    "MediaDecoder::mMediaSeekable (Canonical)")
+  , mMediaSeekableOnlyInBufferedRanges(AbstractThread::MainThread(), false,
+                   "MediaDecoder::mMediaSeekableOnlyInBufferedRanges (Canonical)")
   , mTelemetryReported(false)
 {
   MOZ_COUNT_CTOR(MediaDecoder);
@@ -875,6 +877,7 @@ MediaDecoder::MetadataLoaded(nsAutoPtr<MediaInfo> aInfo,
               aInfo->HasAudio(), aInfo->HasVideo());
 
   SetMediaSeekable(aInfo->mMediaSeekable);
+  SetMediaSeekableOnlyInBufferedRanges(aInfo->mMediaSeekableOnlyInBufferedRanges);
   mInfo = aInfo.forget();
   ConstructMediaTracks();
 
@@ -1377,6 +1380,12 @@ MediaDecoder::SetMediaSeekable(bool aMediaSeekable) {
   mMediaSeekable = aMediaSeekable;
 }
 
+void
+MediaDecoder::SetMediaSeekableOnlyInBufferedRanges(bool aMediaSeekableOnlyInBufferedRanges){
+  MOZ_ASSERT(NS_IsMainThread());
+  mMediaSeekableOnlyInBufferedRanges = aMediaSeekableOnlyInBufferedRanges;
+}
+
 bool
 MediaDecoder::IsTransportSeekable()
 {
@@ -1392,14 +1401,23 @@ MediaDecoder::IsMediaSeekable()
   return mMediaSeekable;
 }
 
+bool
+MediaDecoder::IsMediaSeekableOnlyInBufferedRanges()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  return mMediaSeekableOnlyInBufferedRanges;
+}
+
 media::TimeIntervals
 MediaDecoder::GetSeekable()
 {
   MOZ_ASSERT(NS_IsMainThread());
   // We can seek in buffered range if the media is seekable. Also, we can seek
   // in unbuffered ranges if the transport level is seekable (local file or the
-  // server supports range requests, etc.)
-  if (!IsMediaSeekable()) {
+  // server supports range requests, etc.) or in cue-less WebMs
+  if (IsMediaSeekableOnlyInBufferedRanges()) {
+    return GetBuffered();
+  } else if (!IsMediaSeekable()) {
     return media::TimeIntervals();
   } else if (!IsTransportSeekable()) {
     return GetBuffered();
diff --git a/dom/media/MediaDecoder.h b/dom/media/MediaDecoder.h
index 16cf42ef0c4e..e85fb0cf9ec8 100644
--- a/dom/media/MediaDecoder.h
+++ b/dom/media/MediaDecoder.h
@@ -253,12 +253,17 @@ public:
   // Called from HTMLMediaElement when owner document activity changes
   virtual void SetElementVisibility(bool aIsVisible) {}
 
-  // Set a flag indicating whether seeking is supported
+  // Set a flag indicating whether random seeking is supported
   void SetMediaSeekable(bool aMediaSeekable);
+  // Set a flag indicating whether seeking is supported only in buffered ranges
+  void SetMediaSeekableOnlyInBufferedRanges(bool aMediaSeekableOnlyInBufferedRanges);
 
-  // Returns true if this media supports seeking. False for example for WebM
-  // files without an index and chained ogg files.
+  // Returns true if this media supports random seeking. False for example with
+  // chained ogg files.
   bool IsMediaSeekable();
+  // Returns true if this media supports seeking only in buffered ranges. True
+  // for example in WebMs with no cues
+  bool IsMediaSeekableOnlyInBufferedRanges();
   // Returns true if seeking is supported on a transport level (e.g. the server
   // supports range requests, we are playing a file, etc.).
   bool IsTransportSeekable();
@@ -792,6 +797,9 @@ protected:
   // True if the media is seekable (i.e. supports random access).
   Canonical<bool> mMediaSeekable;
 
+  // True if the media is only seekable within its buffered ranges.
+  Canonical<bool> mMediaSeekableOnlyInBufferedRanges;
+
 public:
   AbstractCanonical<media::NullableTimeUnit>* CanonicalDurationOrNull() override;
   AbstractCanonical<double>* CanonicalVolume() {
@@ -833,6 +841,9 @@ public:
   AbstractCanonical<bool>* CanonicalMediaSeekable() {
     return &mMediaSeekable;
   }
+  AbstractCanonical<bool>* CanonicalMediaSeekableOnlyInBufferedRanges() {
+    return &mMediaSeekableOnlyInBufferedRanges;
+  }
 
 private:
   // Notify owner when the audible state changed
diff --git a/dom/media/MediaDecoderStateMachine.cpp b/dom/media/MediaDecoderStateMachine.cpp
index 7b457cfd69bf..d765321ec7e8 100644
--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -268,6 +268,8 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
                    "MediaDecoderStateMachine::mDecoderPosition (Mirror)"),
   mMediaSeekable(mTaskQueue, true,
                  "MediaDecoderStateMachine::mMediaSeekable (Mirror)"),
+  mMediaSeekableOnlyInBufferedRanges(mTaskQueue, false,
+                 "MediaDecoderStateMachine::mMediaSeekableOnlyInBufferedRanges (Mirror)"),
   mDuration(mTaskQueue, NullableTimeUnit(),
             "MediaDecoderStateMachine::mDuration (Canonical"),
   mIsShutdown(mTaskQueue, false,
@@ -350,6 +352,7 @@ MediaDecoderStateMachine::InitializationTask(MediaDecoder* aDecoder)
   mPlaybackRateReliable.Connect(aDecoder->CanonicalPlaybackRateReliable());
   mDecoderPosition.Connect(aDecoder->CanonicalDecoderPosition());
   mMediaSeekable.Connect(aDecoder->CanonicalMediaSeekable());
+  mMediaSeekableOnlyInBufferedRanges.Connect(aDecoder->CanonicalMediaSeekableOnlyInBufferedRanges());
 
   // Initialize watchers.
   mWatchManager.Watch(mBuffered, &MediaDecoderStateMachine::BufferedRangeUpdated);
@@ -1474,9 +1477,8 @@ MediaDecoderStateMachine::Seek(SeekTarget aTarget)
     return MediaDecoder::SeekPromise::CreateAndReject(/* aIgnored = */ true, __func__);
   }
 
-  // We need to be able to seek both at a transport level and at a media level
-  // to seek.
-  if (!mMediaSeekable) {
+  // We need to be able to seek in some way
+  if (!mMediaSeekable && !mMediaSeekableOnlyInBufferedRanges) {
     DECODER_WARN("Seek() function should not be called on a non-seekable state machine");
     return MediaDecoder::SeekPromise::CreateAndReject(/* aIgnored = */ true, __func__);
   }
@@ -2192,6 +2194,7 @@ MediaDecoderStateMachine::FinishShutdown()
   mPlaybackRateReliable.DisconnectIfConnected();
   mDecoderPosition.DisconnectIfConnected();
   mMediaSeekable.DisconnectIfConnected();
+  mMediaSeekableOnlyInBufferedRanges.DisconnectIfConnected();
 
   mDuration.DisconnectAll();
   mIsShutdown.DisconnectAll();
diff --git a/dom/media/MediaDecoderStateMachine.h b/dom/media/MediaDecoderStateMachine.h
index 2c13d6a2f68a..04a77e656eb1 100644
--- a/dom/media/MediaDecoderStateMachine.h
+++ b/dom/media/MediaDecoderStateMachine.h
@@ -1239,6 +1239,9 @@ private:
   // True if the media is seekable (i.e. supports random access).
   Mirror<bool> mMediaSeekable;
 
+  // True if the media is seekable only in buffered ranges.
+  Mirror<bool> mMediaSeekableOnlyInBufferedRanges;
+
   // Duration of the media. This is guaranteed to be non-null after we finish
   // decoding the first frame.
   Canonical<media::NullableTimeUnit> mDuration;
diff --git a/dom/media/MediaFormatReader.cpp b/dom/media/MediaFormatReader.cpp
index 2af5cfb64e11..895fe7ffd4b3 100644
--- a/dom/media/MediaFormatReader.cpp
+++ b/dom/media/MediaFormatReader.cpp
@@ -337,6 +337,8 @@ MediaFormatReader::OnDemuxerInitDone(nsresult)
   }
 
   mInfo.mMediaSeekable = mDemuxer->IsSeekable();
+  mInfo.mMediaSeekableOnlyInBufferedRanges =
+    mDemuxer->IsSeekableOnlyInBufferedRanges();
 
   if (!videoActive && !audioActive) {
     mMetadataPromise.Reject(ReadMetadataFailureReason::METADATA_ERROR, __func__);
@@ -1428,7 +1430,7 @@ MediaFormatReader::Seek(SeekTarget aTarget, int64_t aUnused)
   MOZ_DIAGNOSTIC_ASSERT(mVideo.mTimeThreshold.isNothing());
   MOZ_DIAGNOSTIC_ASSERT(mAudio.mTimeThreshold.isNothing());
 
-  if (!mInfo.mMediaSeekable) {
+  if (!mInfo.mMediaSeekable && !mInfo.mMediaSeekableOnlyInBufferedRanges) {
     LOG("Seek() END (Unseekable)");
     return SeekPromise::CreateAndReject(NS_ERROR_FAILURE, __func__);
   }
diff --git a/dom/media/MediaInfo.h b/dom/media/MediaInfo.h
index a1427d6221d8..217e15f81a8e 100644
--- a/dom/media/MediaInfo.h
+++ b/dom/media/MediaInfo.h
@@ -417,6 +417,9 @@ public:
   // True if the media is seekable (i.e. supports random access).
   bool mMediaSeekable = true;
 
+  // True if the media is only seekable within its buffered ranges.
+  bool mMediaSeekableOnlyInBufferedRanges = false;
+
   EncryptionInfo mCrypto;
 };
 
diff --git a/dom/media/webm/WebMDemuxer.cpp b/dom/media/webm/WebMDemuxer.cpp
index d01496e6f217..20a703879c1d 100644
--- a/dom/media/webm/WebMDemuxer.cpp
+++ b/dom/media/webm/WebMDemuxer.cpp
@@ -431,7 +431,13 @@ WebMDemuxer::ReadMetadata()
 bool
 WebMDemuxer::IsSeekable() const
 {
-  return mContext;
+  return mContext && nestegg_has_cues(mContext);
+}
+
+bool
+WebMDemuxer::IsSeekableOnlyInBufferedRanges() const
+{
+  return mContext && !nestegg_has_cues(mContext);
 }
 
 void
diff --git a/dom/media/webm/WebMDemuxer.h b/dom/media/webm/WebMDemuxer.h
index d1d9a2c61856..a06415e93319 100644
--- a/dom/media/webm/WebMDemuxer.h
+++ b/dom/media/webm/WebMDemuxer.h
@@ -98,6 +98,8 @@ public:
 
   bool IsSeekable() const override;
 
+  bool IsSeekableOnlyInBufferedRanges() const override;
+
   UniquePtr<EncryptionInfo> GetCrypto() override;
 
   bool GetOffsetForTime(uint64_t aTime, int64_t* aOffset);