I'm not exactly how to describe what I'm thinking of - but it's something I think would be quite handy for doing a quick check on formatting and such. This could most likely already be done with some auto-magical CSS, but having it as a universal function would be quite nice and less laborious.
The 'book view' would have a background colour for each tag type (and perhaps class, if you were to be fancy). So paragraphs show up with a light blue background, spans within them show up with a light green background, allowing you to spot pointless tags (and as such get around to getting rid of their redundant CSS at the same time).
I have a burning hatred for the way some MSO exported epubs have nested spans as whole paragraphs, each having their own class etc. While regex fixes that quickly, there are a number of other such similar things which I end up finding later on.
It works well in my head at least! No doubt some shortcoming elsewhere
