CSS Tools

Diagnostic CSS
A set of styles to call eye-watering attention to bad markup, missing or empty attributes, and so on. In both advanced CSS and iE-friendly versions!
Reset CSS
Smooths out many browser inconsistencies by explicitly assigning values to things like element margins and padding, forcing all elements to have the same font size, and so on. You can find resets in most CSS frameworks, and they're a great starting point for creating your own "baseline" styles.