:fullscreen

:fullscreen {
  /* ... */
}

<div class="element">
  <h1><code>:fullscreen</code> pseudo-class demo</h1>

  <p>
    This demo uses the <code>:fullscreen</code> pseudo-class to automatically
    change the background color of the <code>.element</code> div.
  </p>

  <p>
    Normally, the background is light yellow. In fullscreen mode, the background
    is light pink.
  </p>

  <button class="toggle">Toggle Fullscreen</button>
</div>

.element {
  background-color: lightyellow;
}

.element:fullscreen {
  background-color: lightpink;
}

document.querySelector(".toggle").addEventListener("click", (event) => {
  if (document.fullscreenElement) {
    // If there is a fullscreen element, exit full screen.
    document.exitFullscreen();
    return;
  }
  // Make the .element div fullscreen.
  document.querySelector(".element").requestFullscreen();
});

.element {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  font-family: sans-serif;
  padding: 1.2em;
}