<a href="#" class="menu-toggle">Menu</a>
<nav class="mobile-nav">
  <ul>
    <li><a href="#">Home</a></li>
    <li><a href="#">About</a></li>
    <li><a href="#">Contact</a></li>
  </ul>
</nav>

@import url(https://fonts.googleapis.com/css2?family=Rubik);

body {
  font-family: "Rubik", serif;
  font-size: 1.2rem;
  line-height: 1.6;
}

.mobile-nav {
  display: none; 
}

.menu-toggle {
  background-color: #333;
  color: #fff;
  padding: .5rem 1rem;
  text-decoration: none;
}

.menu-toggle:focus-within + .mobile-nav {
  display: block; 
}