pre.line-numbers {
  counter-reset: line;
  position: relative;
  padding-left: 2em;
}

pre.line-numbers code {
  counter-increment: line;
  display: block;
}

pre.line-numbers code::before {
  content: counter(line);
  display: inline-block;
  width: 2.5em;
  padding-right: 0.5em;
  margin-right: 0.5em;
  color: #888;
  text-align: right;
  border-right: 1px solid #ddd;
}