Originally no, and nowadays it seems like it's a result of that tech debt: Youtube did a frontend rewrite using an early spec of web components, that only Chrome ever fully implemented, and they used Polymer to polyfill other browsers with the idea that they'd implement the spec later and get a performance boost. But the spec changed, so they never did, and Youtube is still sending the Polymer-based version to other browsers - even though from other people's tests it looks like Youtube is capable of sending something that doesn't require the polyfill if it thinks Chrome is on the other end.
(This was the state as of a couple years ago, I don't know if there's been further developments since)
