Some questions about Qss
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I think a lot of developers use qss (among developers usnig qt of course). Maybe not a lot of apps use huge amount of qss, but small qss stylings are needed in big part of apps.
About second question. Widgets are marked as done moudle and so no huge new features will be avail there. But if you miss something in qss you can develop it by yourself and request merge into qt5 (and you even can break binary compatibility in this case if you need some really huge feature that does not fit current interfaces).