Protected media element sources

Hello,

there's a slight problem with how Encrypted Media is handled by other web
standards, if the user agent is unable to access the content, because the
CDM is an external application/library.

The easiest case is the case of the Canvas 2D context trying to draw a
protected video, so I'm trying to solve this issue first and extrapolate
from there. On the Canvas2DContext mailing list, I've asked about it and
they think that it would probably be best if media elements in general
would include attributes indicating protected content, that the 2D context
and other standards could reference, instead of defining "if the element is
a video element and the element is protected with EME and the CDM is
indicating that the content can not be drawn to canvas" over and over again.

There really aren't a lot of requirements for these attributes... about the
only thing that would have to be decided is how many... do we need one for
audio? A separate for subtitles or meta data? Or is it reasonable to assume
that if  any one channel is protected, then an unprotected second channel
wouldn't be much use anyway?

4.8.10 Media elements

readonly attribute boolean videoProtected = false;
readonly attribute boolean audioProtected = false;

The videoProtected may be set by any implementation indicating to other
APIs and script access that the content of the frame, including visual
information such as subtitles will not be available for anything but direct
drawing to the screen. The attribute may change value at any given time,
depending on the result of video decoding process.

The description of the audioProtectes attribute is mostly identical, but
deals with audio processing.

Seems like a useful set of attributes, but since video would also limit
access to subtitles, there might be unintended and unnecessary
accessibility issues.

Does anybody see any other issues?

--hans