Another idea, which in modern browsers does stretch the .breakout only to the width of the browser window:
body, html {
overflow-x: hidden;
margin: 0;
padding: 0;
}
div {
padding:0.5em;
}
.container {
width:300px;
background-color:rgba(255,255,0,0.5);
margin:auto;
}
.breakout {
margin:1em -100%; /* old browsers fallback */
margin:1em calc(50% - 50vw);
background-color:rgba(255,0,255,0.5)
}<div class="container">
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.
<div class="breakout">
Break out of container
</div>
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.
</div>Edit: The one and only Chris Coyier explains Full Width Containers in Limited Width Parents, which is somewhat similiar.