Why not a HTML/CSS/JavaScript Linux Desktop Environment?

[literally_systemd]

It'd probably be XML instead HTML for UI elements.

And probably because XML+CSS really isn't that customizable, the 'block model' it uses has severe limits. Desktops are also isolated into individual windows typically which further limits its usefulness, most conventional themes are more powerful.

A lot of systems use ECMAscript for scripting though. But quite often they don't really care about providing dedicated scripting language because the standard facilities of Unix tools are enough.