fallback-content-1.html

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<!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";

      // Insertion point will match nothing and use fallback content.

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

      shadowDiv.appendChild(slot);

      // Append three nodes as children to use as fallback content.

      slot.innerHTML = '<span style="background-color: orange">Hello</span> <span style="background-color: green">World</span>';

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

      shadowRoot.appendChild(shadowDiv);

  </script>

</head>

<body onload="tweak()">

<div id="outer"></div>

</body>

</html>