<section>
  <div class="btn btn__10">
    <a href="#">1-10.四角から角丸</a>
  </div>
</section>
section{
  display: flex;
  margin:2rem;
  justify-content: center;
  align-items: center;
}
.btn a {
  display: inline-block;
  text-decoration: none;
  border: 2px solid #333;
  color: #333;
  padding: .7rem 1.5rem;
  transition: .2s;
}

.btn.btn__10 a {
  transition: .5s;
}

.btn.btn__10 a:hover {
  border-radius: 50px;
}

External CSS

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External JavaScript

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