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The syntax for lengths is a number immediately followed by a unit identifier.
You can use lengths for the value of several style properties (width, height, left, top, right, bottom, font-size, etc.). Some properties allow negative length values, but if a negative length value is set on a property that does not support it, the declaration is dropped.
There are two types of length units:
1. Relative length units - Relative length units specify lengths relative to other measures. Pages using relative length units can be scaled more easily from one device to another.
2. Absolute length units - Absolute length units define absolute measurements which are useful if the physical properties of the output device are known.


Unit Description
Relative length units
em The font-size of the relevant element.
ex The x-height of the relevant element's font. This is commonly the height of the lowercase 'x'.
px Pixels, relative to the viewing device. The px unit identifies a simple pixel, although for printers and very high resolution screens it may identify multiple pixels.
Absolute length units
in inches (1 inch is equal to 2.54cm)
cm centimeters
mm millimeters
pt points (1 point is equal to 1/72 inches)
pc picas (1 pica is equal to 12 point)

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