The basic aim of CSS is to offer hints and aids for the direct translation
of a document and its elements to differing physical media. There are times
when the information contained in a document's structure is not enough to
display the intended content. In HTML, the various list structures are a
perfect example - the bullets and iterative/hierarchical numbers that are
rendered before the items in the list are not explicitly present in the
source document. This content is automatically generated by the browser
when rendering an element from the source document.
CSS2 extends this concept of automatic content generation to allow content
(strings, quotes, list generators or objects) to be rendered before or after
any element. Generated content includes aural rendering as well; auditory
icons can offer powerful usability enhancements for a listener.
Official Reference: CSS Level 2, Section 12.4, 12.5