I wasn't familiar with the ie expressions, so thanks for that tip, as well as
the note about the borders. I don't need a border now, but it's a good thing to
remember.

The syntax of the expression is not much like actual javascript, so I assume
that
document.body.clientWidth < 599 ? "600px" : "100%"
would be translated roughly as:
if the element is less than 599 make it 600px, otherwise make it 100%. Weird
that they choose those characters for separators since they aren't similar to
any javascript I have written. It's also weird that the 599 doesn't have a px
and lacks quotes.

I also put this line of CSS in an IE conditional comment so the code will
validate.
Thanks for the !