style.css

:root {
  --letter-width: 200px;
  --letter-height: 128px;
}
body {
  height: 100vh;
  width: 100vw;
  display: flex;
  justify-content: center;
  align-items: center;
}

/* evelop */
#letter {
  width: var(--letter-width);
  height: var(--letter-height);
  background-color: #bfedfd;
  position: relative;
  box-shadow: 
    0px 4px 3px 3px #44879e,
    0px 4px 3px 2px #8bcfe6,
    0px 4px 3px 4px #dcf6ff;
  cursor: pointer;
  animation-name: jump;
  animation-duration: 0.6s;
  animation-timing-function: ease-out;
  animation-iteration-count: infinite;
  z-index: 0;
}

/* letter animation */
@keyframes jump {
  0% {
    transform: translateY(0);
  }
  100% {
    transform: translateY(-30px);
  }
}

#letter:hover {
  animation-name: shake;
}

@keyframes shake {
  0% {
    transform: translate(0, 0);
  }
  20% {
    transform: translateY(-30px) rotate(-10deg);
  }
  40% {
    transform: translateY(-30px) rotate(10deg);
  }
  100% {
    transform: translate(0, 0);
  }
}

#letter.open:hover {
  animation-name: none;
}
#letter.open {
  animation-name: none;
}


#letter .frontEnvelope {
  position: absolute;
  border-top: calc(var(--letter-height) / 3 + 20px) solid transparent;
  border-left: calc(var(--letter-width) / 2) solid #c9f1ff;
  border-right: calc(var(--letter-width) / 2) solid #c9f1ff;
  border-bottom: calc(var(--letter-height) / 3 * 2 - 20px) solid #bfedfd;
  z-index: 3;
}

#letter .letterCap {
  position: absolute;
  border-left: calc(var(--letter-width) / 2) solid transparent;
  border-right: calc(var(--letter-width) / 2) solid transparent;
  transform-origin: top;
  transition: 0.3s;
  
}

#letter .letterCap.lowerHalf {
  border-top: calc(var(--letter-height) / 3 + 20px) solid #3b85cf;
  transform: rotateX(0deg);
  transition-delay: 0s;
  z-index: 4;
}

#letter .letterCap.upperHalf {
  border-top: calc(var(--letter-height) / 3 + 20px) solid #3b85cf;
  transform: rotateX(90deg);
  transition-delay: 0.2s;
  z-index: 1;
}

#letter.open .letterCap.lowerHalf {
  transform: rotateX(90deg);
}

#letter.open .letterCap.upperHalf {
  transform: rotateX(180deg);
}


 /* content */
#letter .content {
  position: absolute;
  width: calc(var(--letter-width) - 40px);
  height: 60px;
  box-shadow: 0 2px 3px 3px #cacaca;
  left: 15px;
  text-align: center;
  background-color: #ffffff;
  border-radius: 5px;
  padding: 30px 5px;
  font-size: 2px;
  transition: 0.3s;
  z-index: 2;
}

#letter.open .content {
  transform: translateY(-20px);
  transition-delay: 0.2s;
}

#letter.open .content.active {
  width: 800px;
  height: 600px;
  transform: translate(-250px, -300px);
  transition-delay: 0.6s;
  font-size: 20px;
  z-index: 5;
}