<div class="box"></div>
body {
  padding: 64px;
}

.box {
  width: 50px;
  height: 50px;
  background: #1ab4d3;

  transition-property: border-radius;
  transition-duration: 1s; 
}

.box:hover {
  border-radius: 50%;
}

External CSS

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External JavaScript

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