The <select> element can now be customized with CSS

The fact that I'm disproportionally excited about this probably dates me as an early 2000s web developer. But since selects can do things that you simply cannot recreate in HTML, e.g. have options drop downs that extend outside the viewport boundaries, makes this a really helpful feature.

Now, do autocompletes and tag selectors next...

<input type="datetime-local"> with automatic ISO8601 timezone offsets would be awesome too!

ISO-8601 is not the correct format for serializing local time unless it's in the past.

In my experience, a local datetime picker is going to be used almost exclusively for a future date and time. What you want instead of a timezone offset is a zone ID. That way date and tzdata can handle it properly on the backend.

For this case, I will often ask for the timezone, since that data is often updated on the operating system, but I don't believe it's often for someone to be in the same location and have their timezone change. The software that I've maintained the most over the years (since 2009 up to deploying an update today) requires a login, so I have a client account that includes choosing their timezone. I then use that to convert everything to UTC in the database, and when I retrieve the data, I use it to convert UTC back to their local date/time. I felt that it was the best option, since a user moving to a different timezone should still be able to get dates/times back relative to where they have their current timezone set.