There are two versions of this tester. This version uses DOM interfaces that are already in DOM2, but it only works on Mozilla. The other one works in both MSIE5 and Mozilla, but it may not be DOM2 compliant (depending on what makes it into DOM2).
Note that the results may not be accurate. Read my notes on font sizes and my notes in the aspect ratio testing results for more information. The the aspect ratio testing results also explain how to interpret what you see.
(Back to CSS Information, David Baron)