first <symbol>

This symbol will be prepended to the representation when the counter is negative.

second <symbol>

If present, this symbol will be appended to the representation when the counter is negative.

If the counter value is negative, the symbol provided as value for the descriptor is prepended to the counter representation; and a second symbol if specified, will be appended to the representation. The negative descriptor has effect only if the system value is symbolic, alphabetic, numeric, additive, or extends, if the extended counter style itself uses a negative sign. If the negative descriptor is specified for other systems that don't support negative counter values, then the descriptor is ignored.

negative = 
  <symbol> <symbol>?  

<symbol> = 
  <string>        |
  <image>         |
  <custom-ident>  

<image> = 
  <url>       |
  <gradient>  

<url> = 
  url( <string> <url-modifier>* )  |
  src( <string> <url-modifier>* )  

HTML

<ol class="list" start="-3">
  <li>One</li>
  <li>Two</li>
  <li>Three</li>
  <li>Four</li>
  <li>Five</li>
</ol>

CSS

@counter-style neg {
  system: numeric;
  symbols: "0" "1" "2" "3" "4" "5" "6" "7" "8" "9";
  negative: "(-" ")";
}

.list {
  list-style: neg;
}

Result

• list-style, list-style-image, list-style-position
• symbols(), the functional notation creating anonymous counter styles.