I don't have a Mac that I can try it on but looking at the wiki for iweb it has:
Limitations
iWeb is currently in its third version, and has a limited feature set and some unresolved bugs. Some limitations include:
* Separate CSS files are created for each page, rather than a single stylesheet for the entire site.
* There is no option for more advanced users to directly edit the HTML code of templates. HTML support is limited to small snippets that can be included with the page.
* Password protection is not supported on non-MobileMe user websites.
* Users are limited to the built-in templates, as there is no standard way to create new templates.
* Comments on blog pages are not supported on non-MobileMe user websites.
So I wonder if the second comment is the problem if it only has limited support for HTML that you can include.
Do you have a link to a page that you have created that we could look at to see what it looks like?