The MediaSource spec has an activeSourceBuffers attribute that represents the buffers that provide the selected video track, the enabled audio tracks, and the "showing" or "hidden" text tracks. http://dvcs.w3.org/hg/html-media/raw-file/tip/media-source/media-source.html#dom-activesourcebuffers The current implementation assumes that all added buffers are "active." Once we support track selection, activeSourceBuffers should only return buffers for the selected tracks.
This is no longer valid, it was fixed some time ago.