gecko-dev/layout/reftests/ib-split/insert-into-split-inline-5-shadow.html

<!DOCTYPE html>
<html>
 <head>
  <script>
    customElements.define("custom-element", class extends HTMLElement {
      constructor() {
        super();
        const template = document.getElementById("template");
        const shadowRoot = this.attachShadow({mode: "open"})
          .appendChild(template.content.cloneNode(true));
      }
    });
    function doit() {
      var newNode = document.createElement("span");
      newNode.appendChild(document.createTextNode("Nine"));
      document.getElementById("target").appendChild(newNode);
    }
  </script>
  <style>
   body > custom-element { border: 3px solid blue }
  </style>
 </head>
 <body onload='doit()'>
 <template id="template">
   <!-- Multiple insertion points to force appends to be handled via
       ContentInserted -->
  <slot name="foo"></slot>
  <slot name="bar"></slot>
  <slot></slot>
 </template>
  <custom-element id="target"
  ><span>One</span
  ><span>Two</span
  ><span>Three</span
  ><div>Four</div
  ><div>Five</div
  ><span>Six</span
  ><div>Seven</div
  ><div>Eight</div
 ></custom-element>
 </body>
</html>