That would be (progressive??) XBL. (!)

(- it's kind of behavior extension on tag level, yet has JS - and it's orthogonal, like CSS or XSLT (BTW. see that hack: https://news.ycombinator.com/item?id=41245159), unlike JS which is.. untamed and invasive; i.e. there is video (any) tag but you could (+)DIY not touching the document - like custom playing with MPlayer or VLC as a plugin there for all AV formats or sorting filtering editing whatever, all aside custompacks? :)

- or, what about the other way, like a firewall ??