HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>attachShadow slots</title>
  <style>
    my-article { display: block; margin: 1rem 0; }
  </style>
</head>
<body>
  <my-article>
    <span slot="title">Title text</span>
    <p>Body goes to default slot</p>
  </my-article>
  <pre id="out"></pre>
  <script>
    class MyArticle extends HTMLElement {
      constructor() {
        super();
        this.attachShadow({ mode: "open" });
      }
      connectedCallback() {
        this.shadowRoot.innerHTML =
          "<style>h2 { margin: 0 0 0.5rem; color: #7e22ce; }</style>" +
          "<h2><slot name=\"title\"></slot></h2>" +
          "<div><slot></slot></div>";
      }
    }
    customElements.define("my-article", MyArticle);

    document.getElementById("out").textContent =
      "Title text\nBody goes to default slot";
  </script>
</body>
</html>

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Element.attachShadow — named slots (Try It)