Seems to only work in Chrome, I stared at the "end result" in both Firefox and Safari wondering what the heck was so special about a regular old dropdown menu.
Yeah, this is using a new feature[1] only available in Chrome so far. Mozilla is positive[2] about the specification, so we should see it in Firefox eventually, pending an implementation.
[1]: https://developer.chrome.com/blog/a-customizable-select
[2]: https://github.com/whatwg/html/pull/10548