Game engines don't even go nearly that high-level. I see GUIs implemented in C++ more often than in scripting languages, and when they go that route its usually LUA without any declarative markup whatsoever.

Everything is a tradeoff, if you want the ease of HTML+CSS then be prepared to pay the performance hit, and if you want lighting fast GUIs, be prepared to pay the productivity hit.

What about those of us who prefer to stick with tried and true text, like HackerNews? Fairly simple and effective. No need for a GUI when you can just have a line of text convey the idea much faster.

What's so complicated with the Netflix UI? I've been using the web for quite a while but I'm hard pressed to find any benefit, as a user, in the new Netflix UI. It's not like the old one rocked. But come on, it's a few cover images and text.

In general, probably not. Any optimization the game engine uses could also be used by browser engines (they already use DirectX/OpenGL, for example). CSS is just plain complex.

Weirdly, UI interfaces are also a problem in the game world. Where we want to have a designer or artist design and implement something in a game in a fast way we often resort to using Scaleform[1]. It is flash based tool for games, within the industry it is notorious for how slow and inefficient it is. A simple UI element can often be more costly than a major in-game 3D object.

We are willing to pay this extra cost for these elements because they are often required to be quickly put together and then iterated on over a long period of time in a very dynamic and changing system. Some elements may be moved from Scaleform into a more native implementation but for the most part it is left to be slowly parsed and inefficiently rendered.

Basically, where we need fast changing interfaces that arent necessarily implemented by high skill programmers then we are going to take a performance hit to get the same level of visuals we could get if we had the time and manpower to approach it better.

[1]http://gameware.autodesk.com/scaleform