Because it's a great (quick and easy) way of making sure JavaScript and the browser's internal rendering functions don't run at the same time and cause data rac

Because it's a great (quick and easy) way of making sure JavaScript and the browser's internal rendering functions don't run at the same time and cause data races. Asynchronous callbacks share the same queue as gui tasks, and those tasks can run whilst JS is waiting on an action or an I/O task to finish. As the UI uses the same thread as JS, if JS blocks, this would lock up the whole window (clicking on things would do nothing), so it's important functions don't take too long or suspend the thread. There was an experiment within servo to see if rendering and JS could run in separate threads entirely, but I'm not sure how that went? https://github.com/servo/servo/wiki/Design https://github.com/servo/servo/wiki/Design