• Allows content not in the original document to be displayed
• An example already is <ol> and <ul> producing numbers or bullets
• Every element has a pseudo-element :before and :after which is immediately before and after the element

p.comment:before {content:"Comment: "; color:green}
p.comment:after {content:" (end of Comment)"; color:red}
. . .
<p class="comment">A paragraph</p>

Produces: