<!DOCTYPE HTML>
<html>
<head>
  <script>
    function tweak() {
      // div with style "border: 10px solid green"
      var shadowDiv = document.createElement("div");
      shadowDiv.style.border = "10px solid green";

      var slot = document.createElement("slot");
      shadowDiv.appendChild(slot);

      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
      shadowRoot.appendChild(shadowDiv);
    }
  </script>
</head>
<body onload="tweak()">
<div id="outer">Hello</div>
</body>
</html>