I propose that <source> add a quality, bitrate, or filesize attribute to allow
the UA to decide between multiple streams by choosing the maximum quality file
that it can download within a reasonable amount of time (e.g. it will download
faster than it will play) or based on a user preference (e.g. prefer SD
quality, or always use HD when provided). It should also be possible to
retrieve a list of the <source>s the UA can play in JS, and switch between them
by user action (either a JS call for a custom UI or a dropdown in the builtin
UI), loading the new file and switching to it with minimal skipping. This way,
a site like YouTube, which presents several files in various bitrates and
codecs, can allow the user to choose to use a higher quality without having to
force an src attribute on the video, and a mobile UA that roams from 3G to WiFi
or moves close to a base station can increase the quality of its stream. I
think it fits in well with the purpose of the source element. This is certainly
open for modification, but I think it's a good concept in essence.
