v2 feature: in workers??

It's worth thinking a bit about how we could extend the JS APIs in the future to work in workers. Obviously they don't work as-is because we don't have `Node` and friends.

What would be ideal here is convincing ourselves either that:

- The current API is basically compatible with being put in workers, if we solve the underlying issues around DOM trees. (For example, if we had an opaque `WorkerNode` type which could only be used via a worker version of `DOMWritable`.)

- A worker-based API is so different that it doesn't impact this API much at all. For example, it would be primarily string-based.

I'm not sure how much closure we'll get here, but it's worth considering and giving a good think.

Start by reviewing the existing JavaScript API and its assumptions about Node and DOM trees. Compare whether worker support can preserve the current API or requires a separate string-based design. Done means documenting a clear direction for future worker support, though the issue does not name files or tests.

Written by the indexing model from the issue text.