transition image grayscale on css3

grayscale.css

.grayscale {
  -webkit-filter: grayscale(100%);
  -moz-filter: grayscale(100%);
  -ms-filter: grayscale(100%);
  -o-filter: grayscale(100%);
  filter: grayscale(100%);
  filter: gray; /* IE 6-9 */
  
  -webkit-transition: all 1s;
  -moz-transition: all 1s;
  -o-transition: all 1s;
  transition: all 1s;
}
.grayscale:hover {
  -webkit-filter: none;
  -moz-filter: none;
  -ms-filter: none;
  -o-filter: none;
  filter: none;
  filter: none; /* IE 6-9 */
}