Length units
A length value is formed by an optional + or -, followed by a number, followed by a two-letter abbreviation that indicates the unit.
There are no spaces in a length value
Eg:
1.3 em is not a valid length value, but 1.3em is valid.
A length of 0 does not require the two-letter unit identifier.
Both relative and absolute length units are supported in CSS1.
Relative units give a length relative to another length property, and are preferred since they will better adjust to different media.
Relative units available:
px : pixels, relative to the canvas resolution, is a dot on the computer screen
em : ems, the height of the element's font
Note: 1em is equal to the current font size.
2em means 2 times the size of the current font.
Eg: if an element is displayed with a font of 12 pt, then '2em' is 24 pt.
The 'em' is a very useful unit in CSS, since it can adapt automatically to the font that the reader uses.
ex : x-height, the height of the letter "x"
Absolute length units are highly dependent on the output medium, and so are less useful than relative units.
Absolute units available:
in : inches; 1in=2.54cm
cm : centimeters; 1cm=10mm
mm : millimeters
pt : points; 1pt=1/72in
pc : picas; 1pc=12pt
Percentage units:
A percentage value is formed by an optional + or -, followed by a number, followed by %. There are no spaces in a percentage value.
Percentage values are relative to other values, as defined for each property.
Most often the percentage value is relative to the element's font size.
