[Bug 19531] simplify MIME type capability detection

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

Aaron Colwell <acolwell@chromium.org> changed:

           What    |Removed                     |Added
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                 CC|                            |acolwell@chromium.org

--- Comment #2 from Aaron Colwell <acolwell@chromium.org> ---
(In reply to comment #1)
> http://dvcs.w3.org/hg/html-media/raw-file/tip/media-source/media-source.
> html#byte-stream-formats says
> 
> "Byte stream format specifications based on WebM and the ISO Base Media File
> Format are provided below. If these formats are supported then the byte
> stream formats described below must be supported."
> 
> In other words, the spec requires that if you support WebM in plain <video>,
> then you must also support it using MSE. This means that you can just use
> HTMLMediaElement.canPlayType(), which is synchronous.

That wasn't my original intent when I wrote that. My original intent was "if
you want to support WebM in MSE, then you must follow the rules that are
outlined below". I didn't mean to imply that if canPlayType() indicates WebM
support then the MSE implementation must support the associated bytestream as
well. 

We can make that requirement explicit if people think that is the best path.
I'm a little concerned that this would prevent UA's from incrementally adding
support for different bytestream formats. If a UA supports both WebM & MP4 then
they would have to add MediaSource support for both before exposing any MSE
functionality. Chrome rolled out WebM-only support first and then eventually
added MP4 later. I would assume that other UA's might want to follow a similar
path.

I was planning on responding to this bug with something along the lines of the
following:

partial interface MediaSource {
  static bool isTypeSupported(DOMString type);
};

True would be returned if the UA is able to create a SourceBuffer for the
specified type. This seemed a little more explicit than what canPlayType()
returns.

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