[whatwg] Proposal: Media element - add attributes for discovery of playback rate support

[Peter Carlson (carlsop)]

Problem:
The problem is that the supported playback speeds of a media element may vary per media item (e.g., an on-demand movie) and as supported by the underlying platform. Currently the application has to discover the supported speeds by trial and error or by an alternative non-HTML5 method that links to the media element.

Recommendation:
We suggest that the application should be able to discover the available speeds at which it can play the media element. These speeds provide the allowed playbackRate values. These speeds are provided as attributes of the media element.

Use case:
1. The application acquires a media element.
2. Before playing it, the application discovers the supported playback speeds (or rates) from attributes in the media element. Note that these may vary from media element to media element: they are not just dependent on platform capability.
3. When the user requests a particular playback speed, the application checks to see if that speed is valid.
4. If it is invalid, the application does not change the speed.
5. If it is valid, the application changes to that speed and the application can assume the media playback will actually change to that speed.
End of use case