Designing web sites for mobile devices may not be as straightforward as you think as it is, thanks to hardware manufacturers increasing pixel densities in modern phones and tables. See an example in the image below: a 16px font size is different on Galaxy Tab from a Kindle Fire. Luckily there is a solution although it means keeping tabs with the specifics of each device.
Read on Scott Column's article on A List Apart http://bit.ly/xdWHhC
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DON'T USE PIXELS FOR YOUR FONT SIZE !
+Hadrien Lanneau but the issue is not just font-size but also widths, margins, paddings, etc :)