style.css

body{
    margin: 0;
    padding: 0;
    display: flex;
    justify-content: center;
    height: 100vh;
    align-content: center;
    background-color: peachpuff;
}

#piano{
    height: 290px;
    padding: 90px 30px 0 30px;
    background-color: gray;
    border-radius: 10px;
    border: 1px solid black;
}

.keys {
    position: relative;
}

.white-keys{
    display: flex;
}

.black-keys{
    position: absolute;
    display: flex;
}

.white-key{
    width: 68px;
    height: 280px;
    border-radius: 0 0 5px 5px;
    border: solid #000 2px;
    box-sizing: border-box;
    background-color: #fff;
}

.black-key{
    width: 38px;
    height: 160px;
    border-radius: 0 0 5px 5px;
    box-sizing: border-box;
    position: absolute;
    background-color: #000;
}

.black-key:nth-child(1){
    left: 48px;
}

.black-key:nth-child(2){
    left: 117px;
}

.black-key:nth-child(3){
    left: 253px;
}

.black-key:nth-child(4){
    left: 320px;
}

.black-key:nth-child(5){
    left: 389px;
}

.black-key:nth-child(6){
    left: 525px;
}

.black-key:nth-child(7){
    left: 594px;
}

.black-key:nth-child(8){
    left: 662px;
}

.black-key:nth-child(9){
    left: 797px;
}

.black-key:nth-child(10){
    left: 865px;
}