Okay, now I've tried to play a 320p stream in webm and mp4 from YouTube
(can't link to those as those links I get in keepvid.com expire after some
time, so you'll have to use that tool yourselves), and the webm one streams
smoothly while the mp4 one at times stops to buffer. I've looked at the
codec details of each file to check if perhaps the mp4 file has a larger
bit rate, but it's not even that case (~340kBps for webm compared to
~330kBps for mp4), so indeed it might be related to that bug you mentioned.
