Somebody already has this.
It's not based around HTML so much, but it's based around internet programming technologies.. like javascripting and such.
(this my understanding from listenning to a audio interview, but that was a while ago... so it may or may not be un-completely inaccurate)
It's based around the 'gecko' rendering engine, which is what was created by the Mozilla project for designing applications such as Firefox, Mozilla, Thunderbird, and
Komodo IDE.
For example the windows and the menus and such in Firefox are rendered about the same way that the html page your looking at is rendered. You can use technologies like CSS to modify the look and behavior of the buttons and such on the firefox browser and that's partially were you get all the different themes from. Mozilla.org created the XUL markup language for using the UI stuff and it's based around XML. You can build applications using technics familar to any webprogrammer. (css, javascripting, etc).
The actual operating system is called "Symphony OS". It's a Debian/Knoppix-based operating system with normal Linux kernel and X-windows and all that stuff so it can run any applications and do anything any other Linux-based operating system can.
The difference is the 'Mezzo' interface. It's not based around Gnome or KDE or anything like that, but it is actually a mozilla-based (I beleive) UI that runs on a X server that you can use to access files, launch applications for, and do anything else that you'd normally use a desktop for.
It's a fairly radical departure from normal UI design. It's specificly designed to be completely usefull for the non-computer savy computer user to do basic things in a easy and effecient manner.
Screenshots and other information is aviable
here
They have code, conceptual papers, forums, wikis and all that fun stuff, too.
Right now it's under heavy alpha developement and the current release is called 'Alpha 4'. It's aviable as a 'live cd' linux distro.
This means that it's like Knoppix. The entire operating system, GUI and all, runs directly from your cdrom drive without having to install anything on your harddrive. (although you can probably run it from your harddrive and it will probably run faster that way, if you want to install it, but I am not sure in the case of this specific distro).
It's kinda interesting, check it out.
edit:
I may have been confused about the whole thing. Not sure.... (I realy need to try it out myself)
Here is a wikipedia link describing more aspects of it.
http://en.wikipedia.org/wiki/Symphony_OS