Opening for official review given the resolution in #11824 (comment).

Thanks @tabatkins for already taking a look. @fantasai feel free to lmk if you have any feedback. In the meantime, I'll reach out to the WebDriver folks to see if they have any thoughts on this change.

Adding @shs96c in comment since I wasn't able to add as a direct reviewer.

He is one of the WebDriver spec editors, and said that the change looks good overall, but offered to take a closer look at the content of the change once back from vacation.

He also noted that there is a new version of the WebDriver spec:

"The only comment I would make is that the main focus on the Browser Tools and Testing WG is now WebDriver BiDi. Browser vendors are focusing on implementing that and not the “Classic” spec, so for adoption I’d suggest extending BiDi.

The main difference between the two is that while the original spec used URL templates and the verbs of HTTP, BiDi is event based, passing messages using a websocket. In terms of addding to the spec, I think either approach requires about the same amount of text."

So in the meantime, I'll look at the Bidi version of the spec and make the necessary updates to move over to that model

An update for reviewers (@tabatkins, @fantasai, @shs96c, @svgeesus), I've updated this PR to be based on the webdriver bidi spec. All PR links in the description have been updated.

Based on existing Bidi modules, this seemed to fit nicely into the existing emulation module, which required a draft spec change in w3c/webdriver-bidi#905.

I'll inform the bidi spec owners of these PRs, as well, for review.