I'm working on a new site using an external CSS style sheet, and find that the preview pane only displays the outer DIVs and graphics, but none of the content material.

The pages display normally in IE, Firefox, Safari, and Chrome -- just not within the CCHTML Preview pane. I've tried both the 2008 and 2009 versions.

Any hints about what kind of bug in my code (probably the CSS) would cause this?

We would need to see the code to be able to help. Have you got an URL to share?

Yep links and/or code would help here Netmom. I use an external CSS file without any difficulties so I'm sure it's just some oversite somewhere. Also keep in mind that you do need to save your CSS form everytime you make a change for it to be seen in the Preview Pane. The HTML files don't matter on saving, but the CSS files do. Other than that, we do need a bit more info to help out as Inger suggested.