If I understand everything correctly you are manually implementing support for various selected CSS properties (backgrounds, borders, fonts, whatever), but this doesn’t mean that the whole current CSS3 spec will be supported by the S&Box UI framework.
My question is the following: are you guys currently keeping track of what CSS features/properties are implemented?
It’s probably worth creating a wiki page with everything that can be used even at this point, listing implemented/partially implemented/planned CSS features both for you not to get lost and other developers to be a bit less speculative about what is achievable “just as we do in web” and what isn’t.
Also, what are the priorities on the countless CSS features, what are you willing to support first/second/third of all? What are you certainly not going to support at all?
UPD 4/20/2021: Supported Style Properties - S&box Wiki