how to make things transparent in css code example

Example 1: css how to make background transparent

/* Use RGBA */
background-color: rgba(255, 255, 0, 0.75); /* Transparent Yellow */

Example 2: css opacity example

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

Example 3: transparent background css

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;   
}