Hi Martin
I'm using Mozilla/5.0 (OS/2; U; Warp 4.5; en-US; rv:1.9.3a5pre) Gecko/20100515 SeaMonkey/2.1a2pre (DRY) - no idea what (DRY) means though - on eCS2.0RC6a currently; wil be moving to 2.0GA shortly.
Yes, Seamonkey V2 is HTML5 capable.
I had a bit of fun with the links you provided and the only real problem I had was a system crash when on this page
http://www.benjoffe.com/code/demos/canvascape/ - maybe it was because I jumped and shot at the same time?
However, with video playback it seems to depend on what codec(s) the websites use as to whether a video will play. This build of Seamonkey supports ogg vorbis so vids encoded using that play fine.
Just for fun I joined the youtube html5 beta - and discovered they are using a different codec :-(
The good news may be that the codec I think youtube are using is the free/open codec from Google, WebM (aka VP8)
http://www.webmproject.org/ and that reading some posts in the mozilla ng indicates work is underway porting that to OS/2 Mozilla browsers :-)
This may be a good page to find out a bit about HTML5
http://www.html5video.org/ - and the Demos link might be fun :-)
Regards
Pete