css opacity

.opacity30 {
  opacity: 0.3;
  filter: alpha(opacity=30); /* For IE8 and earlier */
}

.div{
  opacity: 0.8; /* 1 means fully visible, 0 means invisible */
}

background: rgba(255, 255, 255, 0.25);

h1 {background-color:rgba(255,0,0,0.3);}

div {
  background-color: yellow;
}

.light {
  opacity: 0.2; /* Der Text kann kaum über dem Hintergrund gelesen werden */
}

.medium {
  opacity: 0.5; /* Der Text ist besser über dem Hintergrund lesbar */
}

.heavy {
  opacity: 0.9; /* Der Text ist sehr deutlich über dem Hintergrund lesbar */
}

div {
  width: 200px;
  height: 200px;
  display: block;
  position: relative;
}

div::after {
  content: "";
  background: url(image.jpg);
  opacity: 0.5;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  position: absolute;
  z-index: -1;   
}

img {
  opacity: 0.5;
} 

/* Answer to: "transparent text css" */

/*
  Keep in mind, this will make your text almost impossible to read.

  There's multiple ways to make your text transparent, below are a
  bunch of examples of each way you can make your text transparent.
  Note: Some of the examples below will also hide the element as
  well.
*/

div {
  color: transparent;
}

div {
  visiblity: hidden;
}

div {
  display: none;
}

div {
  opacity: 0;
}

div {
  position: absolute;
  z-index: -1;
}

img {
  opacity: 0.5;
}