Something that has been bothering me is I find myself specifying heights, widths, margin, etc in em, while I typically specify border in px. Maybe this is bad practice, but it's what I've been doing.
The trouble is when trying to figure out how big something is, I can add up the em, and I can add up the pixels, but I didn't know how to combine them.
Today I learned that you can combine them with the calc() function.
calc( 10em + 5px ); will spit out the number.
It's only supported in bleeding edge browsers, but it's a nice feature.
The big gotcha is that you must have a space surrounding the mathematical operator, or it won't get parsed.