It's built in because the browser is best equipped to provide this feature universally for HTML <video> elements and (if necessary) lie to the page about whethe

It's built in because the browser is best equipped to provide this feature universally for HTML <video> elements and (if necessary) lie to the page about whether it's currently active. I was kind of skeptical about it but given that in practice all it does is display a small popout icon on video mouse-over, it's not really that annoying.