* {
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}

#box {
    position: relative;
    border: 3px solid gray;
}

.bloc {
    position: absolute;
    border: 1px solid black;
    border-radius: 4px;
    color: white;
    width: 80px;
    height: 80px;
    line-height: 80px;
    text-align: center;
    display: none;
}