Consider merging https://chromium.googlesource.com/chromium/blink/+/edf5d89706a067cce8891ffe2e24005a6e076031 HTMLKeygenElement was manually managing the UserAgent ShadowRoot instead of using the "framework" with didAddUserAgentShadowRoot. It also created a subclass of HTMLSelectElement just to set the pseudoId on it to "-webkit-keygen-select". We can remove that and then make HTMLSelectElement FINAL.
We should just remove this comment in https://bugs.webkit.org/show_bug.cgi?id=167018 instead.