css how to make background transparent

1/* Use RGBA */
2background-color: rgba(255, 255, 0, 0.75); /* Transparent Yellow */

1.opacity30 {
2  opacity: 0.3;
3  filter: alpha(opacity=30); /* For IE8 and earlier */
4}

1div {
2 opacity:25% ; 
3}

1div {
2  width: 200px;
3  height: 200px;
4  display: block;
5  position: relative;
6}
7
8div::after {
9  content: "";
10  background: url(image.jpg);
11  opacity: 0.5;
12  top: 0;
13  left: 0;
14  bottom: 0;
15  right: 0;
16  position: absolute;
17  z-index: -1;   
18}