It is basicaly not. Just converting HTML to texture with some tricky proxying. The reason is to be able to do that http://pixelscommander.com/polygon/htmlgl/demo/filters.html

What he means is that is exactly what the browser does already. It's just slower because it handles more of the standard.

It rasterizes the DOM.

Browser rasterizes based on it`s assumptions and you cant control it completely which is a bottleneck for performance optimization. Also how would you apply displacement filter on that content?

It's also faster because it's native, not JavaScript.