css/css-scoping/host-specificity.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Test: the selector inside :host() affects specificity</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="help" href="https://drafts.csswg.org/css-scoping/#host-selector">
<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/1915">
<link rel="help" href="https://bugzil.la/1454165">
<link rel="match" href="reference/green-box.html">
<p>Test passes if you see a single 100px by 100px green box below.</p>
<div id="host"></div>
<script>
  host.attachShadow({ mode: 'open' }).innerHTML = `
    <style>
      :host(#host) {
        width: 100px;
        height: 100px;
        background: green;
      }
      :host {
        background: red;
      }
    </style>
    <slot></slot>
  `;
</script>