[b=StackCard] {
    position:absolute;
    background:silver;
    top:0;
    left:0;
    right:0;
    bottom:0;
    padding:32px;
    margin-left:0;
    border:solid blue;
    -moz-transition:all 0.5s ease;
}

[b=StackCard].StackCard-out {
    background:red;
    margin-left:100%;
}