Enabling the Media Source API requires assigning the mediaSourceURL property to the src attribute. This requires running JavaScript to enable the media source functionality. 

It would be nice to specify a Media Source specific URL scheme so that a more declarative approach could be used. This would enable us to reuse the fallback behavior already supported by the <source>.

For example:

<video src='x-media-source:abcd' type='video/webm; codecs="vorbis,vp8"' />

<video>
  <source src='x-media-source:abcd' type='video/webm; codecs="vorbis,vp8"' />
  <source src="x-media-source:efg" type="video/mp4" />
  <source src='video.webm' type='video/webm; codecs="vorbis,vp8"' />
  <source src="video.mp4" type="video/mp4" />
</video>


The path for the x-media-source URL is necessary to differentiate which source actually gets selected. The path could also be used to specify an initial SourceID. This would have the added benefit of avoiding the need for an sourceAddId() call in the common case of only needing a single SourceID.