I recently pushed out a change that will use the iframe style embedding for YouTube videos. So it should use the HTML5 video if you have opted into YouTube's HTML5 trial.

The mobile version of Google’s video-sharing website received an upgrade Thursday. The new m.youtube.com has a bunch of new features, including high-quality video playback in the browser using ...

Sticking a couple big ones up Adobe’s way, Vimeo’s embeddable Universal Player allows for playback of Vimeo videos on iPads and other non-Flash-supporting devices, using HTML5 —but also ...

YouTube yesterday announced it has begun using HTML5 video by default for all playback on its website, marking a substantial step in its gradual move ...

In short, to start using HTML5 by default, YouTube had a lot more to consider than just its main use case (playing videos).