From http://www.html5rocks.com/en/tutorials/masking/adobe/?redirect_from_locale=zh

new_gist_file.css

img:hover {
  clip-path: polygon(0px 208px, 146.5px 207px, 147px 141.2px, ...);
  animate: star 3s;
}

@keyframes star {
  0% {
    clip-path: polygon(0px 208px, 146.5px 207px, 147px 141.2px, ...);
  },
  100% {
    clip-path: polygon(0px 208px, 146.5px 207px, 147px 141.2px, ...);
  }
}