Web Design in a Nutshell, 3rd Edition

Absolute Versus Relative Sizes

Before diving into the details of specifying type size, it is worth pausing to clarify the difference between absolute and relative sizes. Absolute sizes have predefined meanings or an understood real-world equivalent. In CSS, absolute values may be expressed as keywords, such as small or x-large (discussed next) or by using absolute length values, such as cm (centimeter), in (inch), or pt (point, 1/72 of an inch).

Relative sizes, on the other hand, are based on the size of something else, like the parent element or the em measurement of the text (see the sidebar "A Word About Ems“). Relative values, such as em and percentages, are generally preferred for web text for reasons that are covered in the upcoming sections.