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?

>>11
WebSockets are currently removed from all browsers due to ``security''
According to caniuse.com, it's just disabled by default. Still, could you elaborate on this supposed ``unresolved protocol-level security issue''^[1]?

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_{References:
[1]: caniuse.com}