class MyElement extends HTMLElement {
  constructor() {
    super();
    this.shadow = this.attachShadow({ mode: "open" });
  }

  connectedCallback() {
    const div = document.createElement("div");
    div.textContent = "I'm in the shadow DOM, my font-family is Bungee Tint, but you cannot see it";
    const button = document.createElement("button")
    button.textContent = "Attach the @font-face rule to the document"
    
    button.addEventListener("click", () => {
      document.adoptedStyleSheets = [fontFace];
      div.textContent = "I'm in the shadow DOM, my font-family is Bungee Tint, now you see it!";
      button.hidden = true;
    })
    

    const fontFace = new CSSStyleSheet();
    fontFace.replaceSync(`@font-face {
      font-family: 'Bungee Tint';
      font-style: normal;
      font-weight: 400;
      font-display: swap;
      src: url(https://fonts.gstatic.com/s/bungeetint/v3/J7abnpl_EGtUEuAJwN9WmotNOj0xpikA.woff2) format('woff2');
      unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
    }`);
    
     const fontFamily = new CSSStyleSheet();
    fontFamily.replaceSync(`* {font-family: 'Bungee Tint'}`);

    this.shadow.adoptedStyleSheets = [fontFace, fontFamily];
    this.shadow.appendChild(div);
    this.shadow.appendChild(button);
  }
}

customElements.define("my-element", MyElement);