Edit in JSFiddle

<header>
<img src="http://demosthenes.info/assets/images/thumbnails/valhalla.jpg" alt>
<img src="http://demosthenes.info/assets/images/thumbnails/beached-viking-ship.jpg" alt>
<img src="http://demosthenes.info/assets/images/thumbnails/water-maiden.jpg" alt>
</header>

header {
    margin: 0;
    max-width: -moz-max-content;
    max-width: -webkit-max-content;    
    width: max-content; 
    border: 4px solid black;
    font-size:0
}