Yes, Adpative bit-rate is currently missing from HTML5.

However, Apple has a method of doing this with entirely standard HTTP and a few metadata files, as this is how you do adaptive bitrate content on the iphone: http://developer.apple.com/iphone/library/documentation/Netw...

The formats and methods Apple provides in the spec for the IPhone can be used for both Live Content and for adaptive bit-rate pre-recorded content.

I hope eventually the iphone http streaming spec or a derivative would get adopted into HTML5 :)