SOURCE

<div class="visual-piano"></div>
<!-- Made by Zvi Kantor - kantorweb.com -->

body{
    display:flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
    min-height: 80vh;
    margin: 0;
}
*{
    font-family: monospace;
    text-align: center;
    text-transform: capitalize;
    user-select: none;
}
.piano-key{
    cursor: pointer;
    outline: none;
    display: inline-flex;
    justify-content: center;
    align-items: flex-end;
    padding-bottom: 1rem;
    height: 7rem;
    width: 2rem;
    font-size: 1.2rem;
    border: 1px solid #000000;
    position: relative;
}

.piano-key_black {
    height: 4rem;
    max-width: 0rem;
    padding: 0;
    border: 0;
    z-index: 99999999;
    vertical-align: top;
}

.piano-key::after{
    content: var(--val);
    display: block;
}

.piano-key_black::after{
    position: absolute;
    background: black;
    width: 1rem;
    height: 4rem;
    color: white;
    display: flex;
    justify-content: center;
    align-items: center;
}

.piano-key[data-playing='true']{
    background: red;
}

.piano-key_black[data-playing='true']::after{
    background: red;
}