Task Queue processing

Hi,
In HTML5 specs it talks about event loop which is responsible for processing events from the task queues. I was wondering which thread can enqueue events? Lets say if there is a UI event will there be a separate thread that will enqueue event and that event will be processed by the event loop? And also how many such threads can be assumed in the system? Can you give some reference from HTML5 specs to that question
Thank you.Regards,Essam