Использование transition

by webref

HTML

<a>Без перехода</a>
<a class="with-transition">С переходом</a>

CSS

a {
  background: lightgrey;
  color: grey;
  margin-right: 10px;
  padding: 5px 10px;
  transition: none;
  cursor: pointer;
  text-decoration: underline;
}

a:hover {
  background: yellow;
  color: red;
}

a.with-transition {
  transition: 1s;
}