<!doctype html>
<link rel="match" href="pseudo-class-defined-print-ref.html">
<style>
  custom-element {
    display: block;
    width: 100px;
    height: 100px;
    background: green;
  }
  custom-element:not(:defined) {
    background: red;
  }
</style>
<custom-element></custom-element>
<script>
  customElements.define("custom-element", class extends HTMLElement { });
</script>