I know that Comet long polling are bad for Web servers because they occupy one thread per connection. So you cannot have a lot of users with persistent connections or your web server will crash.
Is this the same with web sockets in HTML 5?
How could this solve the resource problem if it occupies one thread too per persistent connection?
This assumption is totally untrue. See the answer I gave here for more info. It is (for instance) entirely possible to use IAsyncHttpHandler in IIS to perform long-polling, without using a thread per client.