SockJS

WebSocket technology is catching up, but it will take a while before all browsers support it.
In the meantime there are loads of projects that aim to substitute for WebSockets and enable 'realtime' capabilities for web apps. But all attempts solve only a part of the general problem, and there isn't any single solution that works, is scalable and doesn't require special deployment tricks.
That's why a new project was born: SockJS - yet another WebSocket emulation library, but this time done right.
SockJS has ambitious goals:

• Simple browser-side and server-side APIs, as close to WebSocket API as possible.
• Well documented scaling and load balancing techniques.
• Transports must fully support cross-domain communication.
• Transports must fall back gracefully in case of restrictive proxies.
• Connection establishment should be fast.
• No Flash on the client side, Javascript only.
• Client-side Javascript must be reasonably well tested.
• Additionally the server side code should be simple, in order to lower the cost of writing servers for different languages.

 https://github.com/majek/sockjs-client/wiki/%5BArticle%5D-SockJS:-WebSocket-emulation-done-right