CSS is amazing, but the HTML DOM isn't. It's just so heavy. Maybe if JSON or some similar format could replace HTML/XML, it could be made lighter? Better, I think, would be to use a human readable format like Eno. It's as terse but much easier to read than JSON.
Recent terminals like Alacritty and Kitty have proved OpenGL is the fastest way to handle text rendering. Xray, the potential next generation of Atom text editor rightly seems to follow suit. Meanwhile, Linux distros like Ubuntu have long been using OpenGL for general rendering to great success as seen with compositors like Compiz.It seems OpenGL might be the best technology for any and all rendering needs. If an OS is built on and uses OpenGL for everything, the consistency could yield great results. Text editing, image manipulation and even office apps would be as fast as possible.