<div id="container">
  <div class="box"></div>
</div>
.box{
  width:100px;
  height:100px;
  border:1px solid #000;
  transition-property:height;
  transition-duration:2s;
  margin:0 auto;
}

.box:hover{
  width:200px;
  height:200px;
}

External CSS

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External JavaScript

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