Turns out, not so much. Here's a screenshot of Safari's rendering. That "Highlights" box should be over on the right, not underneath the content.
The layout is based on the layout described in A List Apart: In Search of the Holy Grail, and there were no problems with Safari rendering those pages.
I stripped everything down to the most basic version of CSS that causes the bug. It's at http://blog.case.edu/jms18/examples/mockups/safari-bug.html. If you look at that site in Firefox, it will look like:
In Safari, it will look like:
Is this a known bug for Safari? Do I have to resort to a CSS-hack directed to Safari? I know they exist but would prefer not to.
By the way, this is Safari 2.0. Not sure what 1.3 would render it as. If somebody has a 1.3 version of Safari and can test it, I'd be much obliged.