Yesterday a WordPress patch improved HTML 5 compatibility by adding the following tags to the TinyMCE editor:
- noscript, embed
- menu, samp, header, footer, article, section, hgroup, nac, aside, details, summary, audio, canvas, command, datalist, figcaption, figure, keygen, mark, meter, output, progress, source, time, video, wbr
It also added some extra HTML colours and contains some optimalisations.
This patch will not be applied in WordPress 3.1, so you will have to wait for WordPress 3.2.
Or use some haxx patching like I do.
Source: changeset #17625
YouTube iframe oEmbed (HTML5) using the video element