<!doctype html>
<html>
  <head>
    <style>
      my-element::part(one) {
        color: blue;
      }
      my-element::part(two) {
        color: red;
      }
    </style>
    <script>
      class MyElement extends HTMLElement {
        constructor() {
          super();
          this.attachShadow({mode: 'open'}).innerHTML = `
            <!-- This div is neccessary to trigger the bug -->
            <div>
              <span part="one">Should be Blue</span> |
              <span part="two">Should be Red</span>
            </div>
          `;
        }
      }
      customElements.define('my-element', MyElement);
    </script>
  </head>
  <body>
    <my-element></my-elemetnt>
  </body>
</html>