multiple.css

.multiple-desktop {
  background-size: cover;
  background-position: center;
  background-attachment: fixed;
  background-repeat: no-repeat;
}

/* http://nimbupani.com/using-background-clip-for-text-with-css-fallback.html */
.multiple-desktop-text {
  background: -webkit-linear-gradient(transparent, transparent);
  background: -o-linear-gradient(transparent, transparent);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-size: cover;
  background-position: center;
  background-attachment: fixed;
  background-repeat: no-repeat;
}

.multiple-mobile-wrapper {
  position: relative;
  height: 100%; 
}

/* http://stackoverflow.com/a/23859719/1221082
 * http://stackoverflow.com/a/23795792/1221082
 */
.multiple-mobile {
  /* old browsers */
  clip: rect(0 auto auto 0);
  clip: rect(0, auto, auto, 0);
  /* ios workaround */
  -webkit-mask-image: -webkit-linear-gradient(top, #fff 0%, #fff 100%);
  /* new browsers */
  -webkit-clip-path: polygon(0 0, 100% 0, 100% 100%, 0 100%);
  clip-path: polygon(0 0, 100% 0, 100% 100%, 0 100%);
  overflow: hidden;

  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;

  z-index: -1;
}

.multiple-mobile-content {
  height: 100%;
}

.multiple-mobile:before {
  content: '';
  
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;

  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}