[MSE] Summary of "Resolving Bug 24370" thread and proposed next steps

In the interest of trying to move forward on the MSE bug 24370, I'm going
to try to provide a high level summary of the "Resolving Bug  24370" thread
and propose a way forward. Please forgive me if I accidentally misrepresent
something.

*Summary:*
1. Changes in languages are allowed during the course of a presentation,
but there is some debate about how these should be represented in HTML5.
The majority opinion in the thread appears to prefer immutable language &
kind attributes and track objects being added & removed to reflect language
changes.

2. Currently track kind can't be signalled in ISO-BMFF content. A new
box/atom needs to be
defined<http://lists.w3.org/Archives/Public/public-html-media/2014Apr/0018.html>to
signal this information.

3. No argument or example has been given that requires Javascript to
explicitly change the language or kind of a track. My interpretation of Jim
Ley's comment<http://lists.w3.org/Archives/Public/public-html-media/2014Apr/0015.html>
is
that the application only wants to be notified of changes in the media and
does not need to be able to change the values on the tracks.

4. Bob Lund and David Singer appear to explicitly support removing the
mutable attribute definitions. I believe Silvia Pfeiffer implicitly
supports their removal based on the arguments she has made in the thread. I
also support removal of the mutable attribute definitions.

*Proposed Next Steps:*
1. Remove the mutable attribute definitions for language and kind. I'm
unclear about the process around this sort of change since this is not an
"at risk" feature. I'd hate to have to go through a long Last Call process
again for this.

2. Work with David Singer and other MPEG savy folks to define a new
box/atom for ISO-BMFF to carry track kind information. This definition
could initially live in the ISO-BMFF byte stream format
spec<https://dvcs.w3.org/hg/html-media/raw-file/default/media-source/isobmff-byte-stream-format.html>while
it is waiting to be officially standardized by MPEG.

3. Sylvia and/or Hixie update the HTML specs to reflect the expected
behavior for language & kind changes. I don't have concrete proposals for
this at the moment, but it seems like there has been confusion around the
intended behavior when track changes occur mid-playback and whether or not
the track attributes can change value during playback.


*Please indicate your support or opposition to the summary and proposed
next steps.*

Thank you,
Aaron