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Note that px units must not be used for a tty media type (for instance, character-grid
devices).
Pixels and Accessibility
The classification of pixels as relative units has nothing to do with the concept of
relative units as it??™s defined in the Web Content Accessibility Guidelines (WCAG)
1.0,6 where ???relative??? means that web page elements can scale easily to meet the
needs of the user??”for example, changing the text size in the browser, or changing
the size of the browser window. In WCAG terms, pixels are absolute, as you can??™t
change the size of a pixel from within your browser.
Absolute Units Explained
Table 2.2 identifies the five absolute units that are available to us.
Table 2.2: Absolute Units
General Syntax and
Nomenclature
Description Unit
millimeters mm
centimeters cm
inches in
5 http://www.w3.org/TR/CSS21/syndata.html#length-units
6 http://www.w3.org/TR/WAI-WEBCONTENT/
Description Unit
points pt
picas pc
The Ultimate CSS Reference 32
Millimeters and centimeters are defined in the SI standard.


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