style.css

html {
  font-size: 10px;
  background-size: cover;
}

body,html {
  margin: 0;
  padding: 0;
  font-family: sans-serif;
}

*{
  box-sizing: border-box;
}

.keys {
  display: flex;
  height: 100vh;
  width: 100vw;
  padding-bottom: 10vh;
  align-items: end;
  justify-content: center;
}

.key:first-child {
  border-radius: 2rem 0 0 2rem;
  border-left: .4rem solid black;
}
.key:last-child {
  border-radius: 0 2rem 2rem 0;
  border-right: .4rem solid black;
}

.key {
  z-index: 10;
  border-bottom: .4rem solid black;
  border-top: .4rem solid black;
  font-size: 1.5rem;
  padding: 1rem .5rem;
  transition: all .07s ease;
  width: 10rem;
  text-align: center;
  color: white;
  background: rgba(0,0,0,0.4);
  text-shadow: 0 0 .5rem black;
}

.playing {
  transform: scale(1.1);
  border-color: #ffc600 !important;
  box-shadow: 0 0 1rem #ffc600;
}

kbd {
  display: block;
  font-size: 4rem;
}

.sound {
  font-size: 1.2rem;
  text-transform: uppercase;
  letter-spacing: .1rem;
  color: #ffc600;
}

body {
  overflow: hidden;
}

.bg-img {
  position: absolute;
  top: 50%;
  transform: translateY(-60%);
  left: 0;
  aspect-ratio: 1 / 1;
  min-width: 100vw;
  min-height: 100vh;
  animation-name: hueRotate;
  animation-duration: 1s;
  animation-timing-function: ease-in-out;
  animation-play-state: paused;
  animation-iteration-count: infinite;
}

@keyframes hueRotate {
  0% {
    filter: hue-rotate(0);
    transform: translateY(-60%);
  }
  50% {
    transform: translateY(-40%);
  }
  100% {
    filter: hue-rotate(360deg);
    transform: translateY(-60%);
  }
}

@media screen and (max-width: 800px) {
  .keys {
    gap: 1rem;
    flex-wrap: wrap;
    align-content: end;
  }
  .key {
    border-radius: 1rem !important;
    border: .4rem solid black;
  }
}