Font size in OS X is too big

I'm creating an HTML/CSS site and I have the font size adjusted the way I want it when I look at the site on Firefox in开发者_运维百科 Windows (which is what I'm developing on). But when I look at the site in OS X (either Safari or FF), the font size is too big. Not massively bigger, but bigger than my client wants. But if I reduce the font any more, it's going to be too small for Windows machines.

I'm specifying all font sizes in ems in my CSS doc.

I realize that different browsers and platforms render things differently, and there's no way to control exactly how the page will look. But this seems like a pretty big issue and I'm wondering if there is some simple solution that I should know about. And if not, what is the complicated solution?

Thanks in advance.

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Use pixel values instead of ems.