I actually think you do need to understand rendering logic to some extent to use CSS effectively. For example I have seen many having a hard time understanding

I actually think you do need to understand rendering logic to some extent to use CSS effectively. For example I have seen many having a hard time understanding why it is trivially easy to align an element to the top of the screen but tricky to align something to the bottom of the screen - something which would be symmetric and equally simple in a typical application GUI framework. But understanding how layouts are generated makes this clear.