Should this be an RFC?

• This is not a substantial change

Which package is this a feature request for?

Lit Core (lit / lit-html / lit-element / reactive-element)

Description

How hard would it be to add a lifecycle method or promise that lets us know:

• when all defined LitElements that are descendants of this LitElement have been rendered and
• are also in an updateComplete state?

Alternatives and Workarounds

Running into this issue has made me more aware that LitElements are basically async which is why statefulness is more-or-less required. However, standard HTML is rendered and ready (from a code standpoint) after it has been appended to the DOM, and offering this promise would allow interoperability with that modality of working with elements and DOM trees.