#box_a {
    position: absolute;
    top: 10px;
    right: 10px;
    bottom: 10px;
    left: 10px;
    background-color:deepskyblue;
}
#box_b {
    position: absolute;
    top: 20px;
    right: 20px;
    bottom: 20px;
    left:20px;
    background: orangered;
}
/* ---------- Botones ---------------- 
.next {
    border-radius: 200px 200px 200px 200px;
    border: 5px rgb(232,78,26) solid;
    margin-left: 50%;
    text-decoration: none;
    color: white;
    font-family: sans-serif;
    font-size: 20px;
    background-color: rgb(232,78,26);
    box-shadow: 2px 2px 2px black;
}
.next:hover{
    background-color: darkred;
    border: 5px darkred solid;
}*/