Codly

html css
img{
filter: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg'><filter id='grayscale'><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+, Firefox on Android */
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-ms-filter: grayscale(100%);
-o-filter: grayscale(100%);
filter: grayscale(100%);
filter: gray;
}
img:hover{
filter: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg'><filter id='grayscale'><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%);
-moz-filter: grayscale(0%);
-ms-filter: grayscale(0%);
-o-filter: grayscale(0%);
filter: grayscale(0%);
}