Subtle animation for underlined navigation links - https://www.shopify.com/partners/blog/text-animation-css

animated-menu-items.scss

li a{
  position: relative;
  
  &:before {
    content: "" ;
    position: absolute;
    width: 100%;
    height: 2px;
    bottom: 0;
    left: 0;
    background-color: #000000;
    visibility: visible;
    transform: scaleX(0);
    transition: all 0.3s;
  }  


  &:hover:before {
    visibility: visible;
    transform: scaleX(0.75);
  }
  
}