After coding in Ruby for a while, and omitting semicolons, it's hard to get back in the habit of doing do when writing CSS. I couldn't figure out how to get a CSS to take effect, and it ends up that I didn't put a semicolon down after "width: 250px".
Problems are always entirely stupid after you figure out what the source is. However, this has led me to wonder about the design of HTML and CSS. HTML is now criticized for being to loose and easy-going with users that use it with errors. However, we forget that without that ease-of-use, and ease-of-forgiveness, HTML wouldn't have had the widespread adoption that it does today. We might still be using GOPHER (maybe).
But I don't think there are the equivalents of CSS checkers. How do you verify visual aesthetics? Maybe it can check for things like overlap between boxes, in different sizes. If general visual aethetics could be measured, then it might be possible to have a Visual CSS evaluator. One can argue that human creativity has a range of aethetics, and that it would be impossible to capture it algorithmically or mathematically. However, I think basics of aesthetics could be captured, because despite the variability of aethetics, they all share commonalities such as symmetry.
And yet, I hope that if something like this ever comes about, it won't be seen as a standard of aesthetics, but more of a tool to run automagically, so that humans won't have to look at it one by one.