Long on my todo list is to add a real C or C++ library css parser but I have not found one that I like. If I do I will add it. Until we do, we have to live with the Sigil css parser which was designed in css2 epub2 days and unfortunately not my code at all so I am not 100% sure I am following it.
So the best we can offer in the case of compound tags is to split them internally and then assume any more complex structure is to be left alone, ie, not offered up as an unused selector of any sort.
I will look into it.
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