CSS3 | Type: CSSRBGA Backgrounds and Shadows

Provides more flexibility and animation potential, particularly with opacity.

.header {
  background: rgba(0, 0, 0, 0.5); /* Black with 50% alpha/opacity */
}
.box {
  background: rgba(255, 255, 255, 0.5); /* White with 50% alpha/opacity */
  -webkit-box-shadow: 0px 3px 12px 0px rgba(0,0,0,0.5); /* Black shadow with 50% alpha/opacity */
  -moz-box-shadow: 0px 3px 12px 0px rgba(0,0,0,0.5);
  box-shadow: 0px 3px 12px 0px rgba(0,0,0,0.5);
}

Reference Links

https://css-tricks.com/the-power-of-rgba/

https://www.cssmatic.com/box-shadow