Counters are, in essence, variables maintained by CSS whose values may be incremented by CSS rules to track how many times they're used.

counter.html

<style>
ul {
  counter-reset: counter;
}
li::before {
  counter-increment: counter;
  content: counters(counter, '.') ' ';
}
</style>
<ul>
  <li>List item</li>
  <li>List item</li>
  <li>
    List item
    <ul>
      <li>List item</li>
      <li>List item</li>
      <li>List item</li>
    </ul>
  </li>
</ul>