On my website (www.aleheads.com) the pagebar is set up in such a way so that there should be equal black space above and below the page names. However, in certain browsers (or with certain versions) that space is not symmetrical and there is more black space beneath the page text than above. Is this something that can be resolved with CSS?
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Personally I combat seeing differences in my website layouts, between web browsers, by starting my projects with the reset stylesheet created by Eric Meyer. This reset stylesheet tries to 0 out all default browser spacing for one thing.
Typically a reset stylesheet is something you start with at the beginning of the website building process though, not sometime during, or after, development. You're likely going to see more problems if you try to implement a reset stylesheet now.
At the point you are at now, it's pretty hard to fight with all the different web browsers adding different default styling to your website.
It would be a lot of extra work depending on how many problems you are noticing: but, you could try doing some php browser detection in your template file, and depending on which browser is detected, add a class to your body tag using wordpress's body_class() function.
For example, if your browser detection script detects Firefox 4, then it could add the class "firefox-4" to the body tag, and then you could apply styles to the .firefox-4 #sidebar that would fix a problem with your #sidebar in firefox 4.