<p class="test"><a href="http://www.pmob.co.uk/temp2/ios-fixed-body.html" target="_blank">Please test outside codepen as codepen seems to break this method </a></p>
<p>Just stuff to scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>
<p>scroll</p>

html,
body {
  margin: 0;
  padding: 0;
}
body {
  background: transparent;
}
body:after {
  content: "";
  position: fixed;
  left: 0;
  top: 0;
  right: 0;
  bottom: 0;
  z-index: -1;
  display: block;
  background: url(https://picsum.photos/id/1015/2000/1500) no-repeat 50% 50%;
  background-size: cover;
}

.test {
  text-align: center;
}
.test a {
  color: #fff;
  font-size: 20px;
  text-decoration: none;
}
.test a:hover {
  text-decoration: underline;
}