So what happens if the page is accessed by users who don't have VIDEO tag support in their browsers? For them, you'll need to provide a way to allow the page to serve up a Flash-encoded version of the video. The good news is that there's a way to do this that is a legitimate behavior of the VIDEO; tag, not a byproduct of some other behavior or a browser-specific quirk.
The way this works is fairly clever: The Flash object is embedded within the VIDEO; tag itself. If the browser supports VIDEO, it attempts to use a stream from the VIDEO tags. If none of those work, the OBJECT tag that points to a Flash player is invoked, and the other elements are automatically ignored.
For an example of this, check out a template called Video for Everybody, created by the folks at Camen Design. It not only falls back elegantly from HTML 5 to Flash but will also work in HTML 4 by degrading to Flash as a fallback, too.
Note that you have to supply the Flash player yourself: You can't just tell Flash to stream a video file. On the plus side, the current edition of Flash streams H.264 natively, so if you already have an H.264 encode you can simply re-use that.
The biggest obstacle the VIDEO tag faces is how each browser chooses to implement it -- what codecs are supported and how they're presented to the end user. It's all in flux, and that means any current implementation might change as the browsers themselves evolve over the next revision and beyond.
It's likely that we'll see two tiers of content that use the VIDEO tag: H.264-encoded content hosted by professional services and portals, such as YouTube and Vimeo, and open-standards content encoded in WebM and Theora, hosted wherever there's bandwidth and space to spare.
The important thing is that they'll exist side by side, powered by the same in-browser technology. If one does emerge as the standard, if only in a de facto fashion, it won't be for lack of competition.
Serdar Yegulalp has been writing about computers and information technology for over 15 years for a variety of publications, including InformationWeek and Windows Magazine.