1em = the current font size. The default font size for browsers is equivalent to 16px. Many sites, Zen Cart included, then reset this in their body tag to 62.5%. that reduces the default font size to 10px and so 1em becomes equal to 10px, 1.2em = 12px etc.
However, if you reset the em for a div, everything inside it will take the new value and if you do it again for something inside the effect will be multiplicative. So for example, if you changed font size to 2em (20px) for a wrapper and then set the font-size for a piece of text inside that wrapper to 1.5em, the resulting text would not reduce to 15px, but increase to 30px.
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