If he had used 2 significant figures (tenths of a percent), his size would be off by less than a dot at 1200 DPI. If he had used 3 significant figures (hundredths of a percent), his size would be off by just about 1 dot at 4800 DPI. Since web browsers tend to round to the nearest pixel on ~100 DPI screens, yes, I’d say 19 signifiant figures is more than a little overkill.
Then again, you never know when you might need to print your web page at billboard size and maintain down-to-the-micron precision.
AFAIK even modern webkit browsers round up at the 1000th's place, so that's where I stop. I'm guessing those are value spit out by some sort of CSS calculator as part of a framework or similar. Having said all that, it's certainly "future proof."
that’s a casualty of using TextMate to parse calculations with Google Calculator. I used to use Calculator.app, but started relying on TM because it’s so easy to use.
Ethan is the fluid grid master and this example shows how powerful it is in conjunction with CSS3 media queries.