One point I'd raise about this is that I think it's fine to be able to indicate progress, be it just to keep the user looking at something while the page is loa

One point I'd raise about this is that I think it's fine to be able to indicate progress, be it just to keep the user looking at something while the page is loading, but I don't like how it seems to duplicate the function of the other progress indicators the browser has. Seeing the rise of client-side web apps, a browser API to show progress directly through the browser's chrome rather than custom UI patters would probably be great and prevent a lot of the fragmentation that things like this might cause. Plus, the less-savvy users might come to expect to see this bar everywhere just because they see it on YouTube, which I think is not a healthy thing for the web in general.