you can do it with web workers
Discussion
Is it reliable? I've used services that claim to be able to do this and often they don't work once backgrounded.
well it depends on your implementation of the web worker haha, but usually you can expect that in modern web browsers most events will be triggered.
there's one thing to look out for tho, web workers are a pain to code for because browsers tend to have very strange caching rules for them by default
you maybe can, I CAN'T!