There may be some confusion with 'smart' HTML5+JS sites
by The Gecko
HTML5 is technically not even required - it just has a lot of nice new features for working with this sort of thing (combined with CSS & JS). You could wrangle a lot of this with HTML4 if you wanted.
Any data it needs, it can just fetch from the server and read as XML or JSON or something. It's a pretty nice way of doing things, because the frontend stuff is all done in your browser, so it can react to your actions immediately to move parts of the layout around, or change what's being displayed. It's nice from the backend side of things, because now you just have to worry about access control, fetching data, and doing very simple formatting as XML or JSON. Much more lightweight than generating entirely new pageviews from scratch everytime a user clicks on a link.
If you've used google apps, you've seen this at work. It's how they get desktop-like applications running in your browser without any special plugins.