style.css

@import url("https://fonts.googleapis.com/css?family=Montserrat:400&display=swap");

*{
  color: #F5F5F5;
  box-sizing: border-box;
  -webkit-tap-highlight-color: transparent;
}

:root{
  font-size: 18px;
  user-select: none;
}

body{
  height: 100vh;
  margin: 0;
  background-color: #3E4750;
  font-family: Montserrat, sans-serif;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

footer{
  padding: .2rem 0;
  font-size: .8rem;
  position: absolute;
  bottom: 0;
}

h1{
  margin: 0 0 6rem 0;
  font-size: 4rem;
  font-weight: 400;
}

p{
  max-width: 90vw;
  margin-top: 4rem;
  font-size: .9rem;
  text-align: center;
}

.piano{
  display: flex;
}

.key{
  width: var(--width);
  height: calc(var(--width) * 4);
}

.white{
  --width: 100px;
  background-color: #FFF;
  border-right: 4px solid #555;
}

.white:last-child{
  border: none;
}

.white.active{
  background-color: #DDD;
}

.black{
  --width: 60px;
  margin: 0 calc(var(--width) / -2);
  background-color: #000;
  z-index: 1;
}

.black.active{
  background-color: #222;
}

@media only screen and (max-width: 768px){
  h1{
    margin: 0 0 3rem 0;
    font-size: 3rem;
  }

  p{
    display: none;
  }

  .key.white{
    --width: 50px;
  }

  .key.black{
    --width: 30px;
  }
}