css/css-scoping/slotted-placeholder.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Scoping Test: ::slotted() allows ::placeholder</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="help" href="https://drafts.csswg.org/css-scoping/#slotted-pseudo">
<link rel="match" href="slotted-placeholder-ref.html">
<div id="host">
  <input placeholder="I should be green">
  <textarea placeholder="I should be green"></textarea>
</div>
<script>
  host.attachShadow({ mode: "open" }).innerHTML = `
    <style>
      ::slotted(*)::placeholder { color: green }
    </style>
    <slot></slot>
  `;
</script>