JSARToolKit is an augmented reality library for JavaScript. It’s an open source library released under the GPL. This is a JavaScript port of FLARToolKit, operating on canvas images and video element contents.
HTML5 Rocks has shared an article: Writing Augmented Reality Application using JSARToolKit, which tells you how to use the JSARToolKit library with the WebRTC getUserMedia API to do augmented reality applications on the web. For rendering, he is using WebGL due to the increased performance it offers. It shows a demo application that puts a 3D model on top of an augmented reality marker in webcam video.