div.content
  a.button.
     Hover Me :D

:root {
  
  --white: #ffffff;
  --purple: #3d0776;
}

.content {
  
  width: 100%;
  height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--purple);
}

.button {
  
  font-size: 2rem;
  font-family: 'Montserrat', sans-serif;
  padding: 1rem;
  position: relative;
  cursor: pointer;
  color: var(--white);
  
  &:after{
        
      content: '';
      background: #fff;
      position: absolute;
      bottom: 0;
      width: 0;
      left: 50%;
      height: 3px;
      transform: translateX(-50%);
      transition: all cubic-bezier(0, 0.96, 0.58, 1.1) 1s;
    }
    
  &:hover {
      
      &:after {
        
        width: 50%;
        transition: all cubic-bezier(0, 0.96, 0.58, 1.1) 1s;
      }
    }
}