I have noticed some interesting behaviour with video playback on various websites, at least with Firefox. Videos on the NZ Herald website, for example, don’t start playing until the tab becomes the active one in the window. Also on YouTube, sometimes after an ad, there is a display inviting you to take part in some kind of survey. This displays for a fixed amount of time, but that time only accumulates while the tab is active -- make it inactive, and the time counter stops until it becomes active again. *But* ... the active tab is still considered to be active even if the window is not the frontmost one. This means if you want to do something else while waiting for a YouTube ad to finish playing (with the sound off, of course) and the extra display to disappear, you just have to do it in an entirely separate window.