The concept of HTML5 video is pretty simple. Web developers have used the <img> tag for a long time. Why not have a similar tag called <video>? Yep, that's what's happening.
Well, it's not specifically that easy just yet. There are multiple video formats that can be used, and not every web browser company agrees which should be "the one." (They all have pretty strong opinions.) What this means for developers is (at least for the time being) we'll have to instruct HTML5 to have more than one source video file.
Yeah, that means you'll have to encode your video more than once as well (at least for now).
For those browsers not supporting HTML5 video, the coder can allow the visitor to be presented with alternate options, such as using a plug-in (or downloading a compatible browser -- that's my preference, personally.)
If you're using a recent, open-source browser, you should be able to view the HTML video demos at many of these sites. Just remember, right now your browser might not support the video format presented by the coder who made the demo. (Demo makers pick and choose which format they wish to encode with.) This is a temporary, unfortunate part of cutting edge demos in a world of many browsers!
Two of my favorite sites that demonstrate HTML5 Video are YouTube and TED.com.
YouTube doesn't default to HTML5, you have to turn it on. Once you watch a video in HTML5 on YouTube, you can right (secondary) click on the playing video and you'll see that it is not using a plug-in, it's really just HTML5!
Here are the instructions for enabling HTML5 video on YouTube.
Here is the TED.com blog explaining how HTML5 users can now access the videos without a plug-in.
