Description

Currently the lit-ssr code has a few TODOs and open questions around how to handle data mismatch between what was rendered on the server and what is hydrated on the client. In some cases we could fairly cheaply detect the mismatch and then re-render the correct value on the client, but at the limit doing this in all cases would require more diffing than we do today and potentially impact hydration performance.

Note that mismatches can occur today when rendering LitElements in top-level lit-html templates that have data-bindings to the element, since the element needs to hold off its initial update/hydration step until hydrate is called on the outer template. This is an instance of needing hydration coordination for custom elements, and should be solved by the "pending hydration" fix in #1388.

Acceptance criteria

Decide on a strategy and break down the work to be done into tasks.