Progressively Enhanced CSS

units example.css

body {
  font-size: 2em;
  /* all browsers apply this */
  
  font-size: 7vw;
  /*
    browsers that understand vw apply this rule.
    browsers that don't understand vw ignore it (and get 2em)
  */
  
  
  background-color: rgb(6,6,6);
  /* all browsers apply this */
  
  background-color: rgba(0,0,0,0.8);
  /*
    browsers that understand rgba apply this rule.
    browsers that don't understand rgba ignore it (and get 666)
  */
}