如何使用CSS使导航栏链接在关闭时不变形？

<nav>
  <ul>
    <li><a href="#">链接1</a></li>
    <li><a href="#">链接2</a></li>
    <li><a href="#">链接3</a></li>
  </ul>
</nav>

nav ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
}

nav li {
  display: inline-block;
}

nav a {
  display: block;
  width: 100px;
  height: 30px;
  text-align: center;
  line-height: 30px;
  background-color: #ccc;
  color: #fff;
  text-decoration: none;
  transition: background-color 0.3s ease;
  box-sizing: border-box;
}

nav a:hover, nav a:active, nav a:focus {
  background-color: #666;
}