AJAX push

We're writing a webserver in our spare time and we are now at the point where we want to explore new web features. What is the best way to do full asynchronous communication with javascripts? Can we just periodically check in javascript if we've received something? Or is this only possible if you do a XMLHttpRequest beforehand? Are there other methods?

Web sockets are awesome because few browsers support them, they never support the same version, and each minor change is a version breaking one.

Or, you could not be a retard and learn how browsers actually use JavaScript.  Hint, script will execute as soon as the </script> element is encountered. XMLHttpRequest is useless for this.