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The basic principle is to use the HTML tags for what they're there for: div tags to create sections in a page, tables to display tabular data, paragraphs for text and headers for (you're probably getting the drift here) headers. This principle is called semantic html. This also means you're separating the layout from your content.
There are methods to create a theme based on divs that look the same in MSIE and Firefox. Just search the web, this has everything to do with clear and valid stylesheets and a small set of MSIE specific workarounds.
Or you can try downloading a CSS theme from
http://xoopsfactory.com/ and look in the theme and css files.
Herko