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17.4. Specifying Values

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

17.4.1. 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. It is preferable to specify sizes using relative measurements such as em and ex. Pixels (px) are acceptable as measurements for elements, but not necessarily for text. See the Section 17.8.2, "Specifying Text Size in Pixels" later in this chapter for the pros and cons.



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