Check out example codes for "background opacity css". It will help you in understanding the concepts better.

Code Example 1

/*
  You have to fake it, as there is no such property
*/

div {
  width: 200px;  /* Image Width */
  height: 200px; /* Image Height */
  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;   
}

Code Example 2

/*fake background opacity css */
<div style="background-color:rgba(0, 0, 0, 0.5);">
   <div>
      Text added.
   </div>
</div>

Code Example 3

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;   
}

Code Example 4

img {
  opacity: 0.5;
}

Code Example 5

img {
  opacity: 0.5;
}

Code Example 6

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

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