Apple’s webkit team killed customised built-in elements by refusing to implement them on ideological grounds, so you get a dripfeed of piecemeal solutions instead.

Apple is actively sabotaging the web for a number of years now. They don't want web apps to be able to compete with native iOS apps.

And they prohibit different browsers from the app store, so users don't even have a choice.

Those reasons are not ideological and there's a new proposal that doesn't have the downsides of the original spec and enables new use cases discovered since: https://github.com/WICG/webcomponents/issues/1029

Nothing with customizable built-ins would give you what the original commenter asked for