.bk1 {
  position: relative;
  display: block;
}
.bk1 span {
    position: absolute;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: #000000ab;
    color: #fff;
    font-size: 1.2em;
    text-align: center;
    display: flex;
    min-height: 30%;
    align-items: center;
    justify-content: center;
}