ACTION-1742 Review of MSE CR

I had an action to review the Candidate Recommendation of the Media Source
Extensions specification [1].  I have performed a cursory review of the
text, with some deep diving on portions where I have some expertise.  In
general, this appears to be a mature API / Data Structure spec with the
(typical) problem that there is little description of what each component
is meant to be.  Reading the API and structures will reveal much, but some
prose to warm up the reader would have been appreciated!

Janina requested that I look in particular at whether the MSE allows for a
ready way to synchronize multiple video streams in order to, for example,
enable sign-language in a companion window along side a video.

The MSE exists *beneath* the layer at which you would implement something
like this.  Or at least, where I would implement something like this.
HTML5 has explicit support for synchronizing multiple video streams through
its video element and the mediagroup attribute, as well as through the
MediaController object.  MSE allows the composition of complex media
streams into MediaSource objects.  Such objects can be attached to a HTML5
MediaController object - indeed, they are expected to be as far as I can
tell.  And since there is explicit support for ensuring that media streams
(video, audio, etc) are synchronized within a mediagroup, my conclusion is
that it should just work.

Having said that, I have personally done no testing of this.  I can imagine
some ways I *could* test it, but that is outside the scope of my review.

I am going to work to complete my detailed review of the document, but my
initial feedback is that this document is designed with A11Y in mind.  I do
not yet see anything that would prohibit the embedding of multiple complex
tracks into a MediaStream, nor the management of multiple such streams in a
synchronized manner via a MediaController object.

-- 
Shane McCarron
Managing Director, Applied Testing and Technology, Inc.