Web Workers are covered. Web Workers communicate with the host page via message passing. If you can modularize your code in such a way to separate the computation needed on a worker thread from say the UI drawing, you won't have a problem with not accessing the DOM from the worker.
Cool, good to hear. I haven't seen a lot on web workers re: the whole 'HTML5' thing (seems like web sockets, local storage and geolocation get most of the pub) so something to look forward to in the book.