Background-image size fixed cover which WORKS on iOS and any another mobile device.

    body:after{
      content:"";
      position:fixed; /* stretch a fixed position to the whole screen */
      top:0;
      height:100vh; /* fix for mobile browser address bar appearing disappearing */
      left:0;
      right:0;
      z-index:-1; /* needed to keep in the background */
      background: url(https://newevolutiondesigns.com/images/freebies/nature-hd-background-9.jpg) center center;
      -webkit-background-size: cover;
      -moz-background-size: cover;
      -o-background-size: cover;
      background-size: cover;
    }
  

Click here for seeing the normal behavior, which doesn't work properly.

See Github repo

More info at stackoverflow.com

Live demo at Doklist.com, Doklist.com.br

A very long paragraph to test how well this solution works.