/* Para usarlo, en el estilo se usa de la forma "animation-name:DivBotonResaltar;animation-duration: 60s;" */
@keyframes DivBotonResaltar {
  0%   {box-shadow: 2px 2px 5px #999;}
  3%  {box-shadow: 3px 3px 12px #444;}
  6%  {box-shadow: 2px 2px 5px #999;}
  100% {box-shadow: 2px 2px 5px #999;}
}

.hover-underline::after,
.hover-underline::before {
  content: '';
  position: absolute;
  width: 100%;
  height: 2px;
  background: linear-gradient(to right, #ff0000, #00ffff);
  bottom: -5px;
  left: 0;
  transform: scaleX(0);
  transform-origin: right;
  transition: transform 0.4s ease-out;
}

.hover-underline::before {
  top: -5px;
  transform-origin: left;
}

.hover-underline:hover::after,
.hover-underline:hover::before {
  transform: scaleX(1);
}