[melky]

очень просто :

<style>
a {
  text-transform: uppercase;
  color: inherit;
  text-decoration: inherit;
  position: relative;
}
a:after {
  width: 100%;
  height: 3px;
  margin-bottom: -9px;
  position: absolute;
  bottom: 0;
  left: 0;
  content: ' ';
  opacity: 0;
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=0)";
  filter: alpha(opacity=0);
  -webkit-transition: opacity 400ms;
  -moz-transition: opacity 400ms;
  -o-transition: opacity 400ms;
  -ms-transition: opacity 400ms;
  transition: opacity 400ms;
}
a:hover:after {
  background-color: #7e848c;
  opacity: 1;
  -ms-filter: none;
  filter: none;
}
</style>

<a href="#">Йа ссылко</a>