It's a bit early for browser support given that it's not usable in Chrome. A better place to look for information about the status is probably here:

https://chromestatus.com/feature/5737365999976448

I think it might be this one?

https://caniuse.com/selectlist

The blog post says that "we're excited to see this feature progress through working groups and standards bodies" but doesn't link to anything that would help figure out which standard this ostensibly implements.