Font size in OS X is too big

Question

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 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.

Answer 1

Use pixel values instead of ems.

Answer 2

This may be very very out of date, but a few years back I used % values that work for most browsers:

px  %
9    56%
10  61%
11  68%
12  74%
13  81%
14  87%
15  94%
16  100%
17  105%
18  111%
19  119%
20  125%
21  131%
22  137%
23  144%
24  150%
25  156%
26  162%
27  168%
28  174%
29  180%
30  186%
31  192%
32  198%
33  204%
34  210%
35  216%
36  222%

So if I wanted 12px sized text I'd use 74%:

.myStyle{
    font-size:74%;
}

At the time, I had actually tested all the % values to see which values worked best across a range of browsers.