<!DOCTYPE html>
<html>
  <head>
    <title>Dynamic test page</title>
    <style>
      body { background-color: #ff00ff }
      input { width: 100%; font-size: 25vh }
    </style>
    <script type="text/javascript">
      function handleInput(e) {
        document.body.style.backgroundColor = e.target.value;
      }
    </script>
  </head>
  <body>
    <input type="text" oninput="handleInput(event)">
  </body>
</html>