Custom elements need an explicit "upgraded" or "resolved" flag

[domenic]

This will be used for CSS to match against.

Questions:

• Is it "upgraded", or "resolved"? See :unresolved vs :not(:upgraded) #418.
• Are there any concerns with setting this flag immediately upon upgrade? (That is, at nanotask time.) It seems like it would cause sync layout, which is usually not great?

[annevk]

@jakearchibald suggested having a promise attached to this state too.

[jakearchibald]

Example use-case: hiding (opacity 0) a component until all the child components it uses in its shadow DOM upgrade

[domenic]

Settled on "defined" in 2432819 since the method is indeed named defineElement. Please see https://w3c.github.io/webcomponents/spec/custom/#dfn-element-defined and use the dfn.js popups to follow links.

If there's interest in a new API for knowing when an element (or tag name, perhaps?) becomes defined, let's file a new issue proposing that.