The fullscreen pseudo-class

The pseudo-class fullscreen sets a style of elements for full-screen browser mode. In order to switch from the standard page display mode to full screen and back, you need to press the F11 key on the keyboard.

Syntax

cssselector:fullscreen {
  
}

Example

Let's click to expand the element with text in full screen mode and recolor its background:

html<div id="txt">
  Full screen
</div>
<p>
  <button onclick="fullScreen()">Click</button>
</p>

cssbody {
  margin: 0;
}
#txt {
  padding: 10px;
  background: #467CBC;
  color: white;
  width: 200px;
}
#txt:-webkit-full-screen {
  background: #E37D76;
}
#txt:-moz-full-screen {
  background: #E37D76;
}
#txt:-ms-fullscreen {
  background: #E37D76;
}

javascriptfunction fullScreen() {
  let elem = document.getElementById('txt'); // Getting the element
  if (elem.webkitRequestFullscreen) elem.webkitRequestFullscreen(); // Chrome, Opera, Safari
  else if (elem.mozRequestFullScreen) elem.mozRequestFullScreen(); // Firefox
  else if (elem.msRequestFullscreen) elem.msRequestFullscreen(); // Internet Explorer, Edge
  else if (elem.requestFullscreen) elem.requestFullscreen();
}

Результат:

See also