basic-shadow-1.html

Enable keyboard shortcuts

<!DOCTYPE HTML>

<html>

<head>

  <script>

    function tweak() {

      // div with style "width:300px; height:100px; background-color:green"

      var shadowDiv = document.createElement("div");

      shadowDiv.style.width = "300px";

      shadowDiv.style.height = "100px";

      shadowDiv.style.backgroundColor = "green";

      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});

      shadowRoot.appendChild(shadowDiv);

  </script>

</head>

<body onload="tweak()">

<div id="outer">

  <div style="width:300px; height:100px; background-color:red;"></div>

</div>

</body>

</html>