I thought it was a static rendering, but then I put in a marquee tag and it moved flawlessly across the screen. Impressive. I can't say I see the practical implementations of this, but I'm sure there's some. But it's impressive nonetheless.
Rendering HTML to WebGL textures would be big, which I think this could do - though probably not as performantly as the browser could. It's currently not possible, partially due to security concerns if I remember right.
Theoretically, now you could render the DOM to a texture instead of to the screen and do some sweet deferred rending.
== Edit ==
Looks like one of Mozilla's Distinguished Engineers blogged about the technique. [1] [2] The creator of emscripten used this technique to render tweets as a texture in the bannabread demo. [3]
[0] https://medium.com/@deathcap1/six-months-of-voxel-js-494be64...
[1] http://robert.ocallahan.org/2011/11/drawing-dom-content-to-c...
[2] https://developer.mozilla.org/en-US/docs/Web/HTML/Canvas/Dra...