[Joeri]

Safari (Webkit) is the hold-out on having this: https://caniuse.com/mdn-api_customelementregistry_builtin_el...

The reasoning for choosing not to support it is here: https://github.com/WebKit/standards-positions/issues/97#issu...

[_heimdall]

Thanks! I was pretty sure Safari was the only major holdout but didn't have a link handy and didn't want to speak out of turn there.

That kind of support is a really tough one to work around with a major browser lacking support. For certain features its not a big deal, but when it comes to extending to get full accessibility support the only substitute is avoiding the feature and rolling it all custom. It really is a shame in my opinion, extending built-ins is extremely powerful.