The prefix descriptor of the @counter-style rule specifies content that will be prepended to the marker representation. If not specified, the default value will be "" (an empty string).
Syntax
/* <symbol> values */ prefix: "ยป"; prefix: "Page "; prefix: url(bullet.png);
Values
<symbol>- 
    
Specifies a
<symbol>that is prepended to the marker representation. It may be a<string>,<image>, or<custom-ident>. 
Examples
Adding a prefix to a counter
HTML
<ul class="index"> <li>The Boy Who Lived</li> <li>The Vanishing Glass</li> <li>The Letters from No One</li> <li>The Keeper of the Keys</li> <li>Diagon Alley</li> </ul>
CSS
@counter-style chapters {
  system: numeric;
  symbols: "0" "1" "2" "3" "4" "5" "6" "7" "8" "9";
  prefix: "Chapter ";
}
.index {
  list-style: chapters;
  padding-left: 15ch;
}
See also
list-style,list-style-image,list-style-positionsymbols(), the functional notation creating anonymous counter styles