css how to change background opacity code example

Example 1: background color with opacity

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

Example 2: css set background opacity

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

Example 3: image opacity in html

img {
  opacity: 0.5;
}