spoiler-alert, css only version. Still doesn't work on firefox. https://github.com/joshbuddy/spoiler-alert

spoiler-css-only.css

spoiler, .spoiler { 
  -webkit-transition: all 125ms linear;
  -moz-transition: all 125ms linear;
  -o-transition: all 125ms linear;
  transition: all 125ms linear;
  
  filter: blur(10px);
  -webkit-filter:blur(10px);
  -moz-filter: blur(10px);
  -o-filter: blur(10px);
  -ms-filter: blur(10px);
}
spoiler:hover, .spoiler:hover {
  filter: blur(5px);
  -webkit-filter:blur(5px);
  -moz-filter: blur(5px);
  -o-filter: blur(5px);
  -ms-filter: blur(5px);
}
spoiler:active, .spoiler:active {
  filter: blur(0px);
  -webkit-filter:blur(0px);
  -moz-filter: blur(0px);
  -o-filter: blur(0px);
  -ms-filter: blur(0px);
}