[Bug 19531] New: simplify MIME type capability detection

https://www.w3.org/Bugs/Public/show_bug.cgi?id=19531

          Priority: P2
            Bug ID: 19531
                CC: mike@w3.org, public-html-media@w3.org
          Assignee: adrianba@microsoft.com
           Summary: simplify MIME type capability detection
        QA Contact: public-html-bugzilla@w3.org
          Severity: enhancement
    Classification: Unclassified
                OS: All
          Reporter: surf5502@gmail.com
          Hardware: All
            Status: NEW
           Version: unspecified
         Component: Media Source Extensions
           Product: HTML WG

MIME type capability detection is really important for a lot of video player
applications, and it has to be fast as to not introduce latency. It is
currently really hard to tell what a MediaSource implementation is capable of
playing. If you want to see if a user agent can play a certain MIME type via
MediaSource you must currently: Create a MediaSource. Set it as the src on a
<video> and add a callback to catch its transition to the "open" state. Once it
is open, add a source buffer with the MIME that you want to play inside of a
try catch statement. Catching a NOT_SUPPORTED_ERR is the current "no."
Otherwise you can assume "yes."

We should keep in mind the scenario where a user has a transcode that they
prefer and that transcode cannot be played back via MediaSource but can be
played back via <video>.

Being able to get a synchronous answer here is probably most important. If
there are strong feelings about reusing canPlayType from HTMLMediaElement that
seems like a fine approach, otherwise we can define something specific for
MediaSource.

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