Creates a custom list counter that accounts for nested list elements.

• Use counter-reset to initialize a variable counter (default 0), the name of which is the value of the attribute (i.e. counter).
• Use counter-increment on the variable counter for each countable element (i.e. each <li>).
• Use counters() to display the value of each variable counter as part of the content of the ::before pseudo-element for each countable element (i.e. each <li>). The second value passed to it ('.') acts as the delimiter for nested counters.

<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>

ul {
  counter-reset: counter;
  list-style: none;
}

li::before {
  counter-increment: counter;
  content: counters(counter, '.') ' ';
}