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Each browser has some differences in how they interpret the CSS you are using, thus the XOOPS design, rightfully so, made accomodations for the different browsers that use the different rendering engines. XOOPS automatically detects the appropriate stylesheet and feeds that one to the browser (try looking at the HEAD information in the source after using both IE and Firefox/Netscape.
Realize, too, there are other CSS files. There is a xoops.css file in the main directory, and there is likely a CSS file in each module, too. These are loaded in order from general to specific: the main one for the CMS (xoops.css), the theme (whatever you specify in your theme), and the module one (whatever is specified in the module and/or in each template as in-line styles). This fits the cascading stylesheets design to a T.
However, most designers design in such a way that they only use stuff that works across all 3 browsers. That being the case, I put this line in the styleMAC and styleNN themes:
@importĀ url(style.css);
Then I don't have to copy and paste the changes in 3 stylesheets.
I've still found that some things don't work as expected at times. If you run into that, then put any browser specific CSS into that specific stylesheet (to load after the main stylesheet, and thus "trumping" it). For instance, in the myReviews module, the "waiting" block uses UL lists which display differently in the Firefox browser (and since I don't have access to a Mac, I have no clue as to how it works there). Thus, I added some specific ones just for the Gecko browser (cause it was easier than figuring out how to make it work for all browsers).
Additionally, for every module that I make, I take all those styles from that style sheet and then paste it into the theme's stylesheet instead. Thus, for my sites that have multiple themes for users to choose from, I only have one place to add/edit those styles and when I add a module, I can make all those changes in one place for each theme and can accomodate those as well as the differences between browsers if any. If the module doesn't specify a CSS it likely uses in-line styles, but you can deal with that pretty easily.
Clear as mud?