Property values in CSS can be defined in a variety of ways:

Length

Relative: em, ex, px. These values are relative to some other value

Absolute: in, cm, mm, pt, pc

Percentage: +20% , -30%, similar to relative

Color

Keywords: black, red, green, yellow, blue, etc

#00ff00 or #0f0

rgb(0, 255, 0) or rgb(0%, 100%, 0%)

URL

url("http://www.w3.org/xyz.gif")

Quotes are optional

Enumeration

inherit, none, solid, block, inline, italic, circle, ....