CSS is simple, but browsers aren't. If you've never had a browser merrily disregard something that's supposed to be the default, you've been very very lucky ;) or just haven't tried enough browsers. It's not always MSIE, either.
The "outset" behavior was from an earlier version of Laura's code when the CSS didn't mention borders at all. The table in question says <border="10" bordercolor="transparent"> and officially there ain't no such property as "bordercolor" so the six browsers were free to do as they pleased. Either by pretending there is such a property, or by tossing in their own default. Technically I guess they were messing with the border-style, which isn't an HTML property at all.
Just went to look it up. If you don't say, borders are supposed to default to your regular text color. (Got that, MSIE 6? Not some shade of gray.) In this case it was set to white for "body", "th" and "td", so a clever browser will instantly spot the loophole. I can't find the rule for overall table borders-- as opposed to cell borders-- but surely it would default to "solid". No idea what "transparent outset" would mean.