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The ex unit is equal to the x-height of the current font, which is normally the height
of a lowercase x, as depicted in Figure 2.2. Interestingly, an x-height is defined even
for fonts that don??™t include a lowercase x.
Figure 2.2: Font size and x-height
In CSS, pixels are considered to be a relative unit because they don??™t correspond to
a fixed physical measure. The CSS2.1 specification provides a lengthy definition
31 General Syntax and Nomenclature
of pixel measurements,5 but the bottom line is that a pixel is relative to the viewing
distance. A standard pixel is 0.26mm (1/96 inch) square.
On an LCD computer monitor, or similar device, a pixel is usually the smallest area
that can be rendered. On high-resolution output devices, such as laser printers or
photosetters, pixels must be rescaled by the user agent so that, for example, a
one-pixel border is approximately as wide on paper as it is on a computer screen.
As such, pixel measurements are relative to the device on which the document is
reproduced.


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