Good luck. Efficient 2D using OpenGL is very difficult. It will never approach the speed of a well designed 2D API. Microsoft gave a lot of insight about the hard it was to get Direct2D to be competitive with GDI/GDI+.
If you do see any performance benefits it likely will be because you support less features than what a modern day browser supports with HTML/CSS. But in my opinion you could probably achieve faster speeds with Canvas in a similar manner.
I agree it will be very difficult. I'm working with WebGL as my day gig and understand how hard it is, as we're working with embedded devices there. That's actually what motivated me to start this project, so web developers can build fast, rich and interactive UI's on the same level as those you see in current generation video game consoles.
It depends on what you are doing. Rendering thousands of sprites with independent rotation and scale is easy. Rendering text is a lot of work, but once you do it you can do all sorts of things with the text. Rendering simple things like bezier curves is very difficult. There are some things in the 2D world that HTML/CSS/GDI/GDI+ are very bad at, compared to OpenGL, and vice versa.
Efficient 2D using OpenGL is very difficult. It will never approach the speed of a well designed 2D API.
All web content on OS X and iOS is rendered using OpenGL. That's not to say it was easy to make efficient, but it definitely outperforms the default CoreGraphics implementation on most content.
It's in very early stages. Feedback is very welcome:
https://github.com/davedx/lustro