FYI: I am likely going to remove the author-provided datalist and fallback UA button in this PR soon based on recent discussions, which will likely reduce the number of changes and complexity.

Should this PR be closed, to avoid confusion? I believe it's been completely superseded by the list at the top of #9799, right?

Yes I'm not keeping this up to date anymore since I decided to split out the PRs and have slight redundancies

I did a review here to try to finish this off.

I pushed several editorial commits. @josepharhar, please review them.

The conformance requirements look reasonable to me.

However, one issue makes me think this is not ready. There are multiple definitions for option HTML element insertion/removing steps, one in the select element section, and one in the option element section. I cannot tell if they are both supposed to run, or if one has become obsolete, or what.

Please update this by merging them, or deleting the obsolete one.

Also, please moving the HTML element insertion/removing/moving steps, for both option and optgroup, into the sections for those elements, instead of keeping them in the select element's section. That can help prevent this sort of problem in the future.

An additional non-blocking note (a followup PR would be fine) is that it would be good to add more examples to the spec that use the new, more complicated forms of select.

Thanks Domenic! I fixed the duplicate insertion/removal steps. I also started a PR for examples: #11465

Great, thank you! Can you also move the optgroup steps to the optgroup element section, instead of having them in the select element section?

Great, thank you! Can you also move the optgroup steps to the optgroup element section, instead of having them in the select element section?

done