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hasUAVisualTransition landed in Firefox 149, making it available across all major engines. As well as having a very catchy name, it lets you avoid doubling up on page transitions. Here's how it works:

Isn't it the first official API exposing "UA" as a term outside of spec texts? (There is the UA-client-hints being incubated, but I don't consider it official).

In terms of "UA" as an initialism, you might be right, but things like the "User-Agent" header and navigator.userAgent have been around 20+ years.

Yes I was referring to the initialism "UA". I remember I had a few complaints over some of my stackoverflow answers where I used the initialism without using the full form at all, and I think someone also complained about such use in the HTML specs, hence I now treat it as not obvious for everyone.