There is a simple solution if you want to use px to specify font size, but still want the usability that em's provide by placing this in your CSS file:
body {
font-size: 62.5%;
}
Now specify you p
(and other) tags like this:
p {
font-size: 0.8em; /* This is equal to 8px */
font-size: 1.0em; /* This is equal to 10px */
font-size: 1.2em; /* This is equal to 12px */
font-size: 2.0em; /* This is equal to 20px */
}
And so on.