I don't think there's any way to get CSS to play a MIDI file, not even with the "content" attribute. That looks like a job for Javascript (good luck! I use it as little as I can)

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What I consider a good use of the hover pseudo-class is something like this (yeah, I would say that, the web site is part of my day job) where the table row is highlighted when the mouse is over it. I was surprised and delighted to find you could use "hover" on a <TR> element, having previously only ever used it on links.

The links turning blue and getting underlined is all controlled by the same hover pseudo-class on a table row combined with an <A> element.