See https://pipwerks.com/2009/02/18/fun-with-canvas-and-mootools-a-rectangle-class/ for an explanation of this page.
Tested in: Firefox 3 (OSX & WinXP), Safari 3 (OSX), Opera 9.6 (OSX), IE6 (WinXP), and IE7 (Vista).
Known issues: IE (VML via excanvas.js) doesn't support gradient strokes.