`:predefined` pseudo-class for built-ins (i.e. not custom elements)

[LeaVerou]

It is currently impossible to match custom elements generically. :not(:defined) matches custom elements that have not yet been defined, but since :defined also matches built-ins, there is no way to target all custom elements.

Admittedly the use cases for this are niche (it came up in a Twitter discussion with a framework author who said they use a different code path when the template contains custom elements), but also it’s a pretty simple addition, so why not.

We could introduce a :predefined pseudo-class to match native elements, authors can simply use :not(:predefined) to match custom elements. Or we could introduce a pseudo-class to directly match custom elements, though I have no idea what to call it 🙃

(Ideally we should eventually introduce more granular selectors to target element by tag prefix and/or suffix but this is a nice quick fix that can be implemented pretty easily, and is useful in its own right.)

[sorvell]

Then to match defined custom elements, you'd need :defined:not(:predefined)? Verbose but explicit and likely uncommon, seems fine.

[sorvell]

Unfortunately, customized built-ins still exist despite Webkit not supporting them, so they probably need to be accounted for here:

1. <input> matches :defined
2. <input is="x-foo"> does not match :defined

Presumably then via this proposal:

1. <input> matches :predefined
2. <input is="x-foo"> does not match :predefined