Simple Label

label.css

a.single-product {
    position: relative;
    overflow: hidden;
    display: block;
}

a.single-product::before {
    content: '售出';
    display: block;
    background-color: #de2424;
    color: #fff;
    font-size: 14px;
    font-weight: bold;
    letter-spacing: 1px;
    transform: rotate(45deg) translate3d(20px,-10px,0);
    width: 90px;
    text-align: center;
    position: absolute;
    right: 0;
    top: 0;
    z-index: 2;
}