css set background opacity

1background: rgba(255, 255, 255, 0.25);

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

1/*
2  You have to fake it, as there is no such property
3*/
4
5div {
6  width: 200px;  /* Image Width */
7  height: 200px; /* Image Height */
8  display: block;
9  position: relative;
10}
11
12div::after {
13  content: "";
14  background: url(image.jpg);
15  opacity: 0.5;
16  top: 0;
17  left: 0;
18  bottom: 0;
19  right: 0;
20  position: absolute;
21  z-index: -1;   
22}

1/*background color opacity with explainations */
2rgba(51, 170, 51, .1)    /*  10% opaque green */ 
3rgba(51, 170, 51, .4)    /*  40% opaque green */ 
4rgba(51, 170, 51, .7)    /*  70% opaque green */ 
5rgba(51, 170, 51,  1)    /* full opaque green */ 
6

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}