HTML5 video has been available in public releases of other browsers for some time now but it was Opera themselves who proposed the idea and provided an early example of the technology.
Just like all the under the hood changes for 10.5, the video implementation has undergone a large change switching from lib[ogg|vorbis|theora] to GStreamer. This change adds a lot more flexibility and much better system integration. GStreamer will run in a separate thread improving responsiveness and audio quality and if GStreamer is available natively on the OS, Opera will use the OS-provided system instead. This should allow for a good playback experience in Linux. Opera believe in open formats and recommend using OGG media but the GStreamer backend will mean that even if a video is not OGG, it may still play as long as GStreamer has the right codec available (Firefox’s HTML5 video implementation will play OGG and only OGG).
Speaking of implementations, Opera have tried their best to meet the ever growing spec and being late to the game (as far as public releases go) brings them a number of benefits. Firefox has already had to deprecate some HTML5 video features due to changes in the spec.
The special build is not available on Mac yet, due to them not having GStreamer available at the moment. (I wonder if they could look at Songbird who use GStreamer).
You have to understand that this goes far beyond just replacing Flash video players. That is the first step, but with video being a first class citizen in the DOM and not hidden in a sandbox, developers can style and play with the video data however they can imagine. You can spin, skew, colourise and even map it onto a 3D cube. Anything else on the page can change or interact with changes in the video. We’ll be able to invent new ways of annotating and commenting on videos, all without the use of Flash, and therefore inherently compatible with any OS and any device, including mobile phones.
Of course, I hope they come out with a Mac build soon so that I can test it out with Video for Everybody. If any readers could submit screenshots for the testing-matrix that’d be appreciated. (I’m particularly interested in Linux as HTML5 video has proven to be unreliable with Firefox 3.5 on Linux; MPlayer-plugin &c. don’t fallback as they should)