<body>
  <main class="container">
    <button class="btn">Submit</button>
  </main>
</body>
body {
  background: lightblue;
}

.container {
  text-align: center;
}

.btn {
  margin: 20px auto;
  padding: 20px;
  width: 180px;
  border-radius: 20px;
  font-size: 1.4em;
  background: papayawhip;
  transition: background .5s ease .25s, width .25s ease;
}

.btn:hover {
  background: azure;
  width: 210px;
}

External CSS

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External JavaScript

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