Couldn't care less about X11
Seriously, who actually uses that for real work?
Now it's funny you should say that but I've looked quite seriously into the potential for this for game UIs but still have the same ol' problem of no libraries out there actually render directly natively to OpenGL. It's all render-to-a-system-texture-and-upload-it-every-frame. This is crap in two ways: 1. the whole copying it to OpenGL every frame thing 2. mixing it with OpenGL rendered parts means readbacks and thus terrible performance.
Certainly. You're easily dealing with 1/10th of the performance you're accustomed to outside browser land (electron etc included) and without all the bells and whistles.
What this means in practice for most people is that you can't get away with writing horrible code - you can still do a lot if you do it smart.
There are complete game frameworks available such as http://phaser.io/
(uses pixi.js to render) (free open source) and http://impactjs.com/
(100$) are probably the "best".
As an example this game uses no framework, just pure html, js and css: http://www.littlewargame.com/
As a bonus when in browser land you get the most advanced/used UI tools at your fingertips along with it.