html.gray {
    -webkit-filter: grayscale(.95);
}

兼容性

html {
    -webkit-filter: grayscale(100%);
    -moz-filter: grayscale(100%);
    -ms-filter: grayscale(100%);
    -o-filter: grayscale(100%);
    filter: grayscale(100%);
    filter: progid:DXImageTransform.Microsoft.BasicImage(grayscale=1);
}