Gtk does use css, but not for the purpose of layout. Their layout rests on the same good, understandable and no-bullshit primitives (GtkContainer subtree) as before the time css-like theming was introduced. Gtk css solved a problem of theme engines (clearlooks, redmond, etc) which had to be done at low-level and thus were less easy to create or modify.
What I honestly didn't understand is why CSS and HTML don't have ui specific namespaces by default that could offer alternatives to div elements that would not be influenced by user agent stylesheets.
I think this was the primary idea behind xhtml back then (when looking at xforms et al) but somehow got lost into some weird hacks to make everything "somehow" run on IE.
Now we have ui and semantics in the same namespace (section, aside, dialog, main, article, footer, header et al) and everybody is just more confused. How should I use dialog, for example? No matter how code turns out, it's always a crappy JS based solution.
To be honest, I love the idea of web components, but I hate that there's no JS free deserialization from html to dom possible.
If you create a solution to advance the semantics of SGML, it's an architecture fail to implement it without the semantic aspects HTML and CSS were designed for.
