Result

SVGを使ってあります

css

.btn {/*ボタン装飾*/
  position: relative;
  display: inline-block;
  width: 166px;
  height: 45px;
  font-size: 14px;
  line-height: 45px;
  text-align: center;
  color: #ffffff;
  cursor: pointer;
  overflow: hidden;
}

.btn svg {/*svg配置*/
  position: absolute;
  top: 0;
  left: 0;
}

.btn svg rect {/*基本図形stroke設定*/
  stroke: #ffffff;
  stroke-width: 6;
  stroke-dasharray: 422, 0;
  stroke-dashoffset: 0;
  transition: all 900ms cubic-bezier(0.19, 1, 0.22, 1);
}

.btn:hover svg rect {/*ホバー時に挙動を変更*/
  stroke-width: 10;
  stroke-dasharray: 110, 312;
  stroke-dashoffset: 183;
}

html

<div class="btn">
  <svg>
    <rect x="0" y="0" fill="none" width="166" height="45" />
  </svg>
  foo bar
</div>

via

UI Button SVG/CSS3 hover effect