There are two types of length units: relative and absolute. Relative length units
specify a length relative to another length property. Style sheets that use relative
units will more easily scale from one medium to another (e.g., from a computer
display to a laser printer).
Relative units are:
em: the ’font-size’ of the relevant font
ex: the ’x-height’ of the relevant font
px: pixels, relative to the viewing device
Example(s):
h1 { margin: 0.5em } /* em */ h1 { margin: 1ex } /* ex */ p { font-size: 12px } /* px */
The ’em’ unit is equal to the computed value of the ’font-size’ property of the
element on which it is used. The exception is when ’em’ occurs in the value of the
’font-size’ property itself, in which case it refers to the font size of the parent element.
It may be used for vertical or horizontal measurement.