Box Model

This tutorial was very helpful for me in understanding <div> layout:

http://www.barelyfitz.com/screencast/html-training/css/positioning/

A simple tool. Input the border radius that you want for an element, and it gives you a live preview. You can set each corner independently, and then it gives you all 3 versions of the CSS code that you will need to insure your corners work across browsers.

http://border-radius.com/

Andrew Hedges’ aspect ratio calculator. Solves for a dimension.

http://andrew.hedges.name/experiments/aspect_ratio/