Whether you specify your element's font size in pixels, points or ems, you can be confident that text will look the same across browsers on Mac and Windows with this simple set of CSS rules.
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How to get consistent font sizes across all browsers
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How to target specific IE browsers using just HTML?
Conditional comments are totally valid markup (for HTML as well as XHTML doctypes) that cause IE browsers running in Windows to parse the contents of the commented markup.