Any workable hack will result a box the same width as this one (400px) in all browsers. Any green-tinged hack validates; any red-tinged hack does not. Apologies to the red-green colorblind among us.
"Box Model Hack" using
"Simplified Box Model Hack" escaping one character (validates unless character in the range a-f is escaped)
escape-encoding a character (validates)
// comments (does not validate)
simple redefining of value (validates)