Cascading Style Sheets/Background

Background properties
There are five properties that control the background of an element:
 * W3C Specification CSS1 CSS2.1 : sets the background color, this will be seen if no background image is specified or the image does not cover the entire element. It will also show through any transparent areas in the image. The color may be set to any CSS color or the keyword.
 * W3C Specification CSS1 CSS2.1 : the image to use as a background.
 * W3C Specification CSS1 CSS2.1 : controls the tiling of the background image (possible values: )
 * W3C Specification CSS1 CSS2.1 : does the background scroll with the page (the default) or is it fixed within the viewport?
 * W3C Specification CSS1 CSS2.1 : places the image relative to the element's bounding box.

You should consider supplying a background color as well as a background image in case the user has disabled images.

Examples:

Shorthand property
W3C Specification CSS1 CSS2.1

The shorthand property can be used to set all five individual background properties in a single declaration, e.g. is equivalent to the previous example.

Note that if some values are omitted from the property the individual properties for the missing values are set to their initial values. This is important to remember if more than one rule is setting the background properties on an element. For example is equivalent to

Advanced Opacity
Opacity is a block level property that effects all the children. To use colors with alpha channels, see RGBA and HSLA.

Opacity in the background property is still a new concept and is not compliant across all web browsers however here is a tiny bit about this function

filter: alpha(opacity=40); -moz-opacity:.40; opacity:.40;

adding the alpha(opacity=40) will add a 40% alpha transparency however as mentioned this still does not work on all browsers, for this to work on Firefox for example the -moz hack has to be used. So -moz-opacity:.40 will obviously do the same thing but this code is only relevant for Firefox users.

W3C Specification CSS3