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{"view":"split-vertical","fontsize":"100","seethrough":"","prefixfree":"1","page":"css"}

document.addEventListener("keydown", function(e) {
  if (e.keyCode == 13) {
    toggleFullScreen();
  }
}, false);

function toggleFullScreen() {
  if (!document.fullscreenElement &&    // alternative standard method
      !document.mozFullScreenElement && !document.webkitFullscreenElement) {  // current working methods
    if (document.documentElement.requestFullscreen) {
      document.documentElement.requestFullscreen();
    } else if (document.documentElement.mozRequestFullScreen) {
      document.documentElement.mozRequestFullScreen();
    } else if (document.documentElement.webkitRequestFullscreen) {
      document.documentElement.webkitRequestFullscreen(Element.ALLOW_KEYBOARD_INPUT);
    }
  } else {
    if (document.cancelFullScreen) {
      document.cancelFullScreen();
    } else if (document.mozCancelFullScreen) {
      document.mozCancelFullScreen();
    } else if (document.webkitCancelFullScreen) {
      document.webkitCancelFullScreen();
    }
  }
}

<img id="img" src="//placekitten.com/800/600">

/**
 * Fullscreen image viewing
 */

* { margin: 0 }

body {
max-width: 700px;
margin: 0 auto;
}

#img { max-width: 100%; }

:-webkit-full-screen body {
  max-width: none;
  background: #000;
}

:-webkit-full-screen #img {
  width: 100%;
  height: 100%;
}