Ok... You want some clarity?
Being a programmer this sort of question always has conditionals.
Are there any DE's or (more generally) DE backends that support building (scripting) or rather modding the UI in HTML and it's associated web languages?
If not what are the existing languages in use, how modifiable are they, and would it be possible to build an interpreter to bridge between a HTML written UI layout and a given language. (from what I understand the current standard appears to be XML so this latter option should be somewhat feesable)
Putting all of the above aside how difficult would it be to do something like this from 'scratch.' Mint's login manager already has the ability to take HTML files for making custom login screens so it is possible to sandwich a Web-Engine on top of the OS and have it talk between the UI and OS components.
That being said @goldilocks mentioned that HTML is really ristrictive in comparison XML. I beg to differ and to that end I built a web page in less than 3 days that mirrors the core UI elements that most DE's use in the way I'd like have them laid out. (non functional of course but it's a start.) :) http://azarel-howard.me/html-desktop/ there are some warts it not being fully polished but the concept is there. (don't forget to mouse over some elements) :)
Hi I've been searching for the past couple of months for a Linux DE that's built on HTML. I found this as I was starting this question http://blogs.gnome.org/alexl/2010/11/23/gtk3-vs-html5/ and it's great but that's GTK via browser.
What I want is to able to build my own DE as such using HTML. I'm currently using Cinnamon in Linux Mint and I can theme it via CSS but I'd like to be able to make my own DE with custom toolbars and such.
Note: I'm not talking about a ground up DE just being able to build the user facing UI (the application switcher [alt+tab],the app menu, toolbars, etc.)
I'd still use a standard window compositor and file manager etc. and it'd still all be linked in with the GTK theming engine but more customizable. :)
EDIT: For Posterity. Let it be known that Deepin 2014 introduced a desktop UI built using HTML, CSS, and JS. It's not exactly stable (plenty of bugs prevent recommendation for use as a daily driver) but the point is that it's possible. And... this forum had the nearsightedness to close my question as unclear. Well I think it's pretty clear now what I was asking and that it (contrary to the responses below) is totally possible.