grayscale

img {
    filter: url("data:image/svg+xml;utf8,<svg xmlns=\'http://www.w3.org/2000/svg\'><filter id=\'grayscale\' filterRes=\'800\'><feColorMatrix type=\'matrix\' values=\'0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0 0 0 1 0\'/></filter></svg>#grayscale"); /* Firefox 10+ */
    filter: gray; /* IE6-9 */
    -webkit-filter: grayscale(100%); /* Chrome 19+ & Safari 6+ */
    -webkit-backface-visibility: hidden; /* Fix for transition flickering */
    -webkit-transition: all 0.5s ease;
       -moz-transition: all 0.5s ease;
        -ms-transition: all 0.5s ease;
         -o-transition: all 0.5s ease;
            transition: all 0.5s ease;
}

img:hover {
    filter: url("data:image/svg+xml;utf8,<svg xmlns=\'http://www.w3.org/2000/svg\'><filter id=\'grayscale\' filterRes=\'800\'><feColorMatrix type=\'matrix\' values=\'1 0 0 0 0, 0 1 0 0 0, 0 0 1 0 0, 0 0 0 1 0\'/></filter></svg>#grayscale");
    -webkit-filter: grayscale(0%);
}