em

One em is equal to the current font size while two ems are equal to to twice the current font size. In Internet Explorer, at the Page menu, there is an option for Text size which allows one to pick options for Largest, Larger, Medium, Smaller, and Smallest. If the size of a font is not concretely defined like so:

font-size: 10px;

 
 

...then its size will change as you change this setting. (By the way, I don't know how to change this setting in other browsers as I don't care.) This allows people to set the fonts to be really big if the web page they are veiwing is accomodating. If you want to be accomodating, but also wish to bias the font-size even as it changes, you can do something like this:

font-size: 1.1em;

 
 

...which is going to make the font 10% larger than the default across all five of the settings found in the Text size option in the Page menu. The pixel value settings in contrast will maintain one fixed size across all five of the settings.