.box {
  max-width: 50%;
  margin: 25px auto;
}

code {
  color: rgb(0, 153, 255);
}
  
a {
  position: relative;
  font-weight: 600;
  text-decoration: none;
  color: rgba(0, 0, 0, 0.4);
  transition: color .3s ease;
}

a::after {
  --scale: 0;
  content: '';
  position: absolute;
  left: 0;
  right: 0;
  top: 100%;
  height: 3px;
  background: #4c81c9;
  -webkit-transform: scaleX(var(--scale));
          transform: scaleX(var(--scale));
  -webkit-transform-origin: var(--x) 50%;
          transform-origin: var(--x) 50%;
  transition: -webkit-transform 0.3s cubic-bezier(0.535, 0.05, 0.355, 1);
  transition: transform 0.3s cubic-bezier(0.535, 0.05, 0.355, 1);
  transition: transform 0.3s cubic-bezier(0.535, 0.05, 0.355, 1), -webkit-transform 0.3s cubic-bezier(0.535, 0.05, 0.355, 1);
}

a:hover {
  color: #4c81c9;
}
a:hover::after {
  --scale: 1;
}