From http://www.w3schools.com (Copyright Refsnes Data)

CSS content Property

Definition

The content property is used with the :before and :after pseudo-elements to generate content to attach before or after a CSS selector.

Inherited: No

CSS properties can also be dynamically changed with a JavaScript.

Scripting Syntax: object.style.content="url(beep.wav)"

In our HTML DOM tutorial you can also find a full Style Object Reference.

p:before
{
content: url("beep.wav")
}

img:before
{
content: attr(alt)
}

Value	Description
string	Defines text content. Default value is ""
url	Defines a url
counter(name) counter(name, list-style-type) counters(name, string) counters(name, string, list-style-type)
attr(X)	Defines an attribute of a selector to be displayed before/after the selector
open-quote
close-quote
no-open-quote
no-close-quote