I would guess most of that is probably Polymer. IIRC the story was, when Youtube was redesigned a while ago they used an early spec for Web Components, that th

I would guess most of that is probably Polymer. IIRC the story was, when Youtube was redesigned a while ago they used an early spec for Web Components, that they were pushing to be standardized and was fully implemented in Chrome. But they had to add Polymer as a polyfill for other browsers. Then the version of Web Components we actually got wasn't entirely compatible with that early spec, so Polymer is still in use for non-Chrome browsers. I don't know how much of that's changed since then, but the complaints are basically the same as when that happened.