Edit in JSFiddle

<a href="#" class="uk-btn" data-btn="Btn Hover Effect">Btn Hover Effect</a>

* {
  box-sizing: border-box;
}

body {
  padding-top: 100px;
  text-align: center;
  background: #222;
}

[data-btn] {
  display: inline-block;
  padding: 12px 18px;
  text-decoration: none;
  position: relative;
  border: 2px solid #fff;
  border-radius: 3px;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  text-align: left;
  color: #fff;
  overflow: hidden;
}
[data-btn]:before, [data-btn]:after {
  content: attr(data-btn);
  padding-top: inherit;
  padding-bottom: inherit;
  white-space: nowrap;
  position: absolute;
  top: 0;
  overflow: hidden;
  color: #222;
  background: #fff;
}
[data-btn]:before {
  left: 0;
  text-indent: 18px;
  width: 0;
}
[data-btn]:after {
  padding-left: inherit;
  padding-right: inherit;
  left: 100%;
  text-indent: calc(-100% - 36px);
  transition: 0.2s ease-in-out;
}
[data-btn]:hover:before {
  width: 100%;
  transition: width 0.2s ease-in-out;
}
[data-btn]:hover:after {
  left: 0;
  text-indent: 0;
  transition: 0s 0.2s ease-in-out;
}