UI means User interface.
It is always good that the appearance of the casino is striking, I don't deny it, based on the things that can be visible are the deposit, withdrawal and some rules, but the main thing that I think the casinos and well all the casinos should have is, that when you enter and register they have to do the KYC, or at least say that the KYC has to be fulfilled so that they can withdraw normally, otherwise they must wait for the KYC to be approved, is that up to now or have I achieved it? in no casino and that I use the casinos that are level 1A, like stake.com, bitcasino.io, so I think that the casinos that are new should warn about these things, so as not to give the players a bad time.
The physical aspect, the first thing we see is good that it surprises, maybe bright colors, something that makes the person get interested, that is good, something that makes the person happy to be there.