Piano

I solved it on May 28, 2021. You can try to beat my record on CSSBattle!

610 chars

<div k>
  <div b></div>
  <div b></div>
  <div></div>
  <div b></div>
  <div b></div>
  <div b></div>
  <div></div>
</div>
<style>
  body {
    background: #998235;
  }
  [k] {
    margin: 100 102;
    display: flex;
    justify-content: center;
    width: 180;
    height: 100;
    background: #191919;
    border-radius: 10px;
  }
  [k] * {
    width: 20;
    height: 70;
    background: white;
    margin: 25 2.5 5 2.5;
    border-radius: 5px;
  }
  [b]::after {
    content: '';
    display: block;
    width: 15;
    height: 35;
    background: #191919;
    position: relative;
    left: 15;
  }
</style>