%

 percentage

 in

inch

 cm

 centimeter

 mm

 millimeter

 em

1em is equal to the current font size. 2em means 2 times the size of the current font. E.g., 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

one ex is the x-height of a font (x-height is usually about half the font-size)

 pt

point (1 pt is the same as 1/72 inch)

 pc

pica (1 pc is the same as 12 points)

 px

pixels (a dot on the computer screen)

color_name

 A color name (e.g. red)

rgb(x,x,x)

 An RGB value (e.g. rgb(255,0,0))

rgb(x%, x%, x%)

An RGB percentage value (e.g. rgb(100%,0%,0%))

#rrggbb

 A HEX number (e.g. #ff0000)