Ronny Orbach

Nice. What about vertical alignment? Adding simliar top properties doesn't seem to work.

Most things are not !important.
Recommended reading: http://coding.smashingmagazine.com/2010/11/02/the-important-css-declaration-how-and-when-to-use-it/

In general, I try to minimize !important use to general states only, e.g. .hidden which should override normal rules.

The difference in actual runtime performance here is so tiny, I doubt if it matters in any real-world use case (who on earth matches millions of selectors, not to mention in a loop over time?).

Just use what's more readable and makes sense in your context. If you nailed the big stuff, tweaked the medium issues and ran out of micro-optimizations, come back to this tip.