.diff-marker {
    position: absolute;
    top: 0;
    left: 0;
    border: 2px solid red;
    padding: 2px;
    opacity: 0;
    filter: alpha(opacity=0);

    -webkit-transition: opacity 1s;
    -moz-transition: opacity 1s;
    -ms-transition: opacity 1s;
    -o-transition: opacity 1s;
    transition: opacity 1s;
}

.diff-marker.on {
    opacity: 1;
    filter: alpha(opacity=100);
}