Docs should note that :defined is not supported

[arthurevans]

May be obvious to some, but was not to me. For native custom elements, can use the :defined pseudo-class to style elements that have been defined (and probably more importantly, :not(:defined) to style elements that haven't been).

Current fix I believe is to manually add and remove an "unresolved" attribute, or equivalent.

[valdrinkoshi]

for completeness, the unresolved trick won't work for those elements that get upgraded lazily (e.g. after some user action). The unresolved attribute styling is done in webcomponentsjs, and the attribute is removed after at WebComponentsReady event.

[notwaldorf]

I think Arthur meant literally removing it manually, in the importHref callback, like how Shop does it.

[snuggs]

Salut! @notwaldorf @valdrinkoshi @arthurevans well done on documenting :defined. We are currently working through some inconsistencies between W3C/CSSOM/WHATWG. :defined and it's pseudo-class definition implementation are also under discussion along with some outstanding CE issues:

WICG/webcomponents#664
WICG/webcomponents#665

This also could shed light on some inconsistencies with Shady re: shadow DOM impl.
WICG/webcomponents#661

Just a heads up. Happy Sunday!