From what I've heard, video support is minimal at best.
From http://diveintohtml5.ep.io/video.html#what-works:
As of this writing, this is the landscape of HTML5 video:
Mozilla Firefox (3.5 and later) supports Theora video and Vorbis audio in an Ogg container. Firefox 4 also supports WebM.
Opera (10.5 and later) supports Theora video and Vorbis audio in an Ogg container. Opera 10.60 also supports WebM.
Google Chrome (3.0 and later) supports Theora video and Vorbis audio in an Ogg container. Google Chrome 6.0 also supports WebM.
Safari on Macs and Windows PCs (3.0 and later) will support anything that QuickTime supports. In theory, you could require your users to install third-party QuickTime plugins. In practice, few users are going to do that. So you’re left with the formats that QuickTime supports “out of the box.” This is a long list, but it does not include WebM, Theora, Vorbis, or the Ogg container. However, QuickTime does ship with support for H.264 video (main profile) and AAC audio in an MP4 container.
Mobile phones like Apple’s iPhone and Google Android phones support H.264 video (baseline profile) and AAC audio (“low complexity” profile) in an MP4 container.
Adobe Flash (9.0.60.184 and later) supports H.264 video (all profiles) and AAC audio (all profiles) in an MP4 container.
Internet Explorer 9 supports all profiles of H.264 video and either AAC or MP3 audio in an MP4 container. It will also play WebM video if you install a third-party codec, which is not installed by default on any version of Windows. IE9 does not support other third-party codecs (unlike Safari, which will play anything QuickTime can play).
Internet Explorer 8 has no HTML5 video support at all, but virtually all Internet Explorer users will have the Adobe Flash plugin. Later in this chapter, I’ll show you how you can use HTML5 video but gracefully fall back to Flash.
As well, you should note this section just below on the same page:
There is no single combination of containers and codecs that works in all HTML5 browsers.
This is not likely to change in the near future.
To make your video watchable across all of these devices and platforms, you’re going to need to encode your video more than once.