Content

The CSS   property automatically generates content to attach before/after a CSS selector (using the  and   pseudo-elements). One or more keywords may be specified for this property, but the content does not actually exist in the document tree; it is generated "on-the-fly." The  property is used with this property to specify the type of rendering box for the generated content.

Values
CSS example:

em:before { content:url("ding.wav"); }