add a way for content to not be displayed until async assets (f.e. link elements) are loaded.

Nobody has claimed this yet.

FUOC is something that people generally wish they didn't have to deal with. To make things easier, perhaps we can add an option that will automatically keep content hidden until async content in a element's ShadowRoot has finished loading.

What we may need is some sort of convention for all elements to follow. For example, maybe elements specify a list of the elements (from their template) that load stuff, then the base Element class can wait for `load` events on all of them, and finally emit a `load` event on the owning element. This setup then allows any higher-up elements to do the same thing, and so on.

Start by examining the base Element class and how elements define or access content in their ShadowRoot. Determine a convention for identifying asynchronously loading elements, waiting for their load events, and propagating a load event to the owning element; done means content remains hidden until those assets finish loading.

Written by the indexing model from the issue text.