Full Browser Width Bars (Inside a fixed width wrapper) using box-shadow

HTML:

<section>
  <h2 class="fullbg">TITLE</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ducimus dolor odit sint nesciunt deleniti dolorem commodi itaque quidem error! Sit officiis repudiandae quae voluptates natus aliquam reprehenderit explicabo dolorem obcaecati.</p>
</section>

CSS:

body {
  background: #ccc;
}
section {
  margin: 0 auto;
  width: 25rem;
  background: white;
  padding: 20px;
}
.fullbg {
  background: black;
  box-shadow: -25rem 0 0 black, 25rem 0 0 black;   /* x offset == element width */
  color: white;
}