They don't. They don't know shit. They can often not answer basic flexbox questions. Not to mention CSS grid questions.
And if you start questioning them about CSS animation performance (paint, composite, layout) they rarely–if ever–even get your point.
Most applicants we get don't even understand basic HTML semantics. I mean, shit, I've seen applicants submit their application projects with EVERYTHING being a div-element. Including links and buttons, which would have onClick events...
The technical aspect of frontend development allows backend developers to bleed into the frontend. "Oh, look, typescript! Finally JS looks like Java! Which means that silly HTML and CSS is going to be easy!"
Buzz off back to the backend, you strong-typed clown who doesn't understand what UX and a11y mean.
I could go on and on about the horror that is typescript. But that's for another day. I know most people love it. I fucking hate it. I'm using it, because I have to, but I never needed it. It literally offers me nothing of value.
But on topic: CSS experts? They are rare. Maybe 1 in ever 30 applicants can actually claim that level of knowledge.