[foms] WebM Manifest

[Timothy B. Terriberry]

> In the case you describe the only drawback is that playback is not as
> perfect as it can theoretically be. But that's expected when using
> adaptive streaming anyway.

The comments I gave before were not meant to be an exhaustive list of 
shortcomings. You also need to either a) know enough about the streams 
in advance to know whether or not such a switch will be successful 
(i.e., if you can't find that information in the manifest, then you'll 
need a full keyframe index, exposed in Javascript, which you would 
otherwise not need), meaning higher startup costs, etc., or b) you can 
try to make such a switch without knowing that it will succeed, and 
frequently download a lot of extra data which must be thrown away when 
you fail. Either way you add a lot of implementation complexity to do 
it. I guess maybe that all still falls under "playback is not as perfect 
as it can theoretically be", but that continues all the way down to, "It 
doesn't play at all."