html {
  background: url(webpage1.jpg) no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

.invisible {
  position: absolute;
  padding: 0;
  margin: 0;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  background:rbga(255,255,255,0.5);
  
}