Aspects of modern HTML/CSS you may not be familiar with

For making UIs, web or not, I always go for HTML+CSS. It can be a bit awkward to interact with native code (e.g. C++), since you'll have to generally do message passing which involves serialisation/deserialisation.

Given that HTML+CSS is really damn powerful as this post makes clear, how do other UI toolkits compare? I've always wanted to do more with Qt, but probably it won't come near HTML+CSS in terms of functionality and flexibility?

Native toolkits now also have CSS (the syntax, not the properties). They also long had an often XML based UI specification language.