Size Matters
If you come from the print world, it's tempting to create "pixel perfect" layouts, where even the font size is fixed. While most browsers allow you to zoom text, "most browsers" add up to about 5% of the browser market; Internet Explorer will not resize text if you specify a font size in pixels or points. Frustrated, squinting visitors are less likely to come back, or give you money.
Specify all your font sizes as percent, or use the keyword sizes:xx-small, x-small, small, medium, large, x-large, xx-large, sothat people who set their own font-size don't get told that you know better.