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10.1. Attribute Value Types

Many element attributes share similar data requirements. For the sake of brevity in the reference listings, this section describes a few common attribute value types in more detail than is possible within each listing. Whenever you see one of these attribute value types associated with an attribute, consult this section for a description of the type.

10.1.1. Length

A length value defines a linear measure of document real estate. Length units may be relative or absolute. A relative unit depends upon variables such as the dot pitch or pixel density of the video display that shows a document. Relative units in CSS are pixels (px), ems (em), and exes (ex). An em is the actual height of the element's font as rendered on a given display device; an ex is the height of a lowercase "x" under the same conditions. The exception to this rule is when em and ex units are used to define the font-size attribute, in which case the units are relative to the font size of the parent element.


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