I am interested in making a game that I currently have a co-op experience, however I'm curious as to the best method of implementing this in HTML5.

I have made games before using straight C sockets, and also with the Net library for SDL.

What are some of my best options for doing this in a canvas-based environment?

At present, all I can come up with are either AJAX/database solutions (with a high refresh rate), or somehow implementing a PHP server that would funnel the data through sockets.

The overall gameplay would be a 2.5D platformer-ish type of game, so both clients would need to be continually updated with player positions, enemy positions, projectiles, environmental data, etc.