he/him 
YouTube's UI is still built with Polymer 1. They did test with Polymer 2 but they're kinda... stuck at v1 to this day
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Lifecycle, element definition, class versus prototype, etc http://polymer-library.polymer-project.org/2.0/docs/about_20#custom-elements-v1 …pic.twitter.com/bkNxgkfJm0
It feels like “nobody implemented our spec verbatim so it’s their fault we had to change and break compatibility”.
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And because YouTube is still working on (...?) migrating from Polymer v1 to v2/3, they are stuck on relying polyfills [1] to run properly on non-Chrome browsers, causing perf issues [1] HTML Imports polyfill (for Polymer 1-2) and Custom Elements v0 polyfill (for Polymer 1)
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