:before and :after definitely works, but it used to be impossible to debug, because they’re not shown in Dev Tools. At least this used to be the case.
This was reported long time ago, but currently I’m in touch with @Erin_Finnegan and I’ll try to find all my sent emails and aggregate what I’ve reported already (appears my emails might’ve not reached Adobe). Most of these emails were about CSS I think, so I’ll try to find some time to get this going
add support for these tags ol and ul to the wish list
:not worked for me. But it is not documented since what version of UXP it works. It is not documented at all.
Ha, making me doubt myself! I’m pretty sure it’s not working in 7.2, I’ll try 7.3 in a moment
I am using it for few months like this already: https://github.com/search?q=repo%3Ajardicc%2Falchemist+%3Anot&type=code
Ok, I take back :not - I made a schoolboy SASS mistake 
.outer {
.inner {
&:not(:first-child) {
// Clever girl
}
}
}
.outer {
.inner {
}
&:not(:first-child) {
// Stupid idiot
}
}
(Discovering this has really improved my mood today!)
Well, regardless of your skills I think it demonstrates another more important issue. Without proper documentation, we have no clue what is supported and we should use ¯\_(ツ)_/¯
I’ve actually found myself pleasantly surprised by the CSS support so far. Coming from CEP and hearing the limitations, I thought it would be less.
The main current complaint I have is that UXP styles override user styles on native HTML elements, such as inputs, sliders, and buttons. It makes it so that we have to build custom javascript alternatives for core elements just to change something like padding.
Also, support for gap property would be nice. Lots of extra :last-child selectors would be cleaned up.
I second that opacity values other than 1 or 0. As a UI designer playing around with opacity instead of a hex value is faster and a better UX.