A few weeks ago I shared my CSS "Ah-ha!" Moment and asked others to share theirs. It was lots of fun hearing what those moments were for people. I wasn't surprised to hear the CSS Zen Garden come up very often, as that was a huge eye-opener for me and countless others I'm sure. I was surprised to hear how many people specifically brought up floats. I'm thinking about doing an in-depth article on that, since I don't think I ever have.
"...realizing you could style the contents of a box differently to the rest of the page" - Rob Lang
"My CSS 'Ah-Ha!', as simple as it might sound, came from understanding that the BROWSER applies IT'S style first and so any CSS you are applying will essentially override those rules" - David Huntley
"Mine was first time I installed Firebug ..." - Francisco