<p>This is some example text. This is a <a href="#">normal link</a>, and this is a <a href="#" class="underline">flashy link</a> using pseudo styles.</p>

.underline {
  text-decoration: none;
  position: relative;
}

.underline:after {
  content: "";
  position: absolute;
  left: 0;
  bottom: -4px;
  z-index: -1;
  height: 12px;
  width: 100%;
  background: yellow;
  transition: background 0.5s;
}

.underline:hover:after {
  background: orange;
}