Web Design in a Nutshell, 2nd Edition

Specifying Values

It is important to use the proper syntax for specifying length and color values in style sheet rules.

Length Units

Table 17-1 lists units of measurements that can be specified in style sheet values.

Table 17-1. Units of measurements for style sheet values

Code	Unit	Description
px	Pixel	Pixel units are relative to the monitor resolution.
pt	Point	A traditional publishing unit of measurement for type. There are approximately 72 points in an inch.
pc	Pica	A traditional publishing unit of measurement equal to 12 points (or 1/6 of an inch).
em	Em	A relative unit of measurement that traditionally equals the width of the capital letter M in the current font. In practical terms, it is equal to the point size of the font(e.g., an em space in 24pt type is 24 points wide).
ex	Ex	A relative unit of measurement which is the height of the letter “x” for that font (approximately half the length of an em).
in	Inches	Standard unit of measurement in the U.S.
mm	Millimeters	Metric measurement.
cm	Centimeters	Metric measurement.

Some values can be specified as percentages that are relative to the font size or bounding box of the element. The following example makes the line height 120% of the element’s font size:

P {line-height: 120%}

Designers should keep in mind that the specific unit measurements listed above (pt, pc, in, mm, and cm) are not good choices for screen design because of the variation in size from monitor to monitor. ...