hmm, Have a gander at the buglist on this page: http://www.macedition.com/cb/ie5macbugs/
As well, the first line: "IE 5 for the Mac has very good support for CSS1, and good partial support of CSS2."
"good partial" prolly wont do it.
I hate to say this but but most xhtml compliant sites, or even just sites that strive to be compliant, arent going to things of beauty on older browsers. A key component of xhtml is seperating style from structure, and that tends to rely on CSS.
A browser that doesnt follow web standards and only has partial support for CSS 2.0 is going to fall short.