This was used to create HTML elements for a shadow dom, but this
is now provided by the scoped-elements mixin via this.createScopedElement().

One way to handle this is enough, so remove this again.
All users have been ported already.

It has new API which we want to use.

Newest Ubuntu switched Firefox to a snap, which breaks karma.

Try to use playwright to download and cache firefox/chromium
and use those for karma.

Compared to puppeteer where we did this some time ago this caches
the browsers globally, and only downloads them when we start the tests.