The :fullscreen pseudo-class lets you configure your stylesheets to automatically adjust the size, style, or layout of content when elements switch back and forth between fullscreen and traditional presentations.

In this example, the color of a button is changed depending on whether or not the document is in fullscreen mode. This is done without needing to specifically apply style changes using JavaScript.

The page's HTML looks like this:

<h1>MDN Web Docs Demo: :fullscreen pseudo-class</h1>

<p>
  This demo uses the <code>:fullscreen</code> pseudo-class to automatically
  change the style of a button used to toggle fullscreen mode on and off,
  entirely using CSS.
</p>

<button id="fs-toggle">Toggle Fullscreen</button>

The <button> with the ID "fs-toggle" will change between pale red and pale green depending on whether or not the document is in fullscreen mode.

The magic happens in the CSS. There are two rules here. The first establishes the background color of the "Toggle Fullscreen Mode" button when the element is not in a fullscreen state. The key is the use of the :not(:fullscreen), which looks for the element to not have the :fullscreen pseudo-class applied to it.

#fs-toggle:not(:fullscreen) {
  background-color: #afa;
}

When the document is in fullscreen mode, the following CSS applies instead, setting the background color to a pale shade of red.

#fs-toggle:fullscreen {
  background-color: #faa;
}

• Fullscreen API
• Guide to the Fullscreen API
• :not
• ::backdrop
• DOM API: Element.requestFullscreen(), Document.exitFullscreen(), Document.fullscreenElement
• allowfullscreen attribute