a line from left to right grow under a navbar

growing_underline.css

&:after {
          display:block;
          content: '';
          border-bottom: solid 3px #ffcd00;
          transform: scaleX(0);
          transition: transform 250ms ease-in-out;
          transform-origin:0 50%;
          box-sizing: border-box;
}

&:hover:after,  &:hover:focus{
                     transform: scaleX(1);
                     box-sizing: border-box;
}