Length values

Length values come in two varieties:

Relative lengths, which vary depending on the computer being used.
Absolute values, which remain constant regardless of the hardware and software being used.

Relative lengths values:

em: Em dash
ex: x-height
px: pixel

Absolute length values:

pt: Point
pc: Picas
mm: millimeters
cm: centimeters
in: Inches

Color values

Color can be written in different way,

#RRGGBB
rgb(R#, G#, B#)
rgb(R%, G%, B%)
name

Percentage

The behavior of percentage value depends on the property being used.

URLs

A uniform resource locator (URL) is the unique address of something on the Web. URL can be written so that it cancel itself

<a href="#">Link</a>

<a href="javascript: void('')">Link</a>

In some situation placing any URL in the href will interfer with the DHTML functions so the latter code will be used instead. These function simply tells the link to do absolutely nothing.