chromium/third_party/blink/web_tests/fast/dom/shadow/new-fallback.html

<!DOCTYPE html>
<html>
<head>
<script src="resources/shadow-dom.js"></script>
</head>
<body>
<div id="sandbox"></div>
<script>
var sandbox = document.getElementById('sandbox');
sandbox.appendChild(
    createDOM('div', {},
              createShadowRoot(
                  createDOM('content', {'select': '#foo'},
                            createDOM('div', {},
                                      document.createTextNode('fallback')))),
              createDOM('div', {},
                        document.createTextNode('host-child'))));
</script>
</body>
</html>