Жирное нижнее подчеркивание

для кнопки например

HTML

<div id="gradient">
  <h2>linear-gradient</h2>
  <div class="menu">
    <div class="nav">
      <ul>
        <li><a href="#">Home</a></li>
        <li><a href="#">Lays</a></li>
        <li><a href="#">Hola</a></li>
        <li><a href="#">Contact</a></li>
      </ul>
    </div>
  </div>
</div>

<div id="border">
  <h2>border-bottom</h2>
  <div class="menu">
    <div class="nav">
      <ul>
        <li><a href="#">Home</a></li>
        <li><a href="#">Lays</a></li>
        <li><a href="#">Hola</a></li>
        <li><a href="#">Contact</a></li>
      </ul>
    </div>
  </div>
</div>

<div id="pseudo">
  <h2>pseudo elements</h2>
  <div class="menu">
    <div class="nav">
      <ul>
        <li><a href="#">Home</a></li>
        <li><a href="#">Lays</a></li>
        <li><a href="#">Hola</a></li>
        <li><a href="#">Contact</a></li>
      </ul>
    </div>
  </div>
</div>

<div id="box-shadow">
  <h2>box-shadow</h2>
  <div class="menu">
    <div class="nav">
      <ul>
        <li><a href="#">Home</a></li>
        <li><a href="#">Lays</a></li>
        <li><a href="#">Hola</a></li>
        <li><a href="#">Contact</a></li>
      </ul>
    </div>
  </div>
</div>

SCSS

* {
  margin: 0;
  padding: 0;
}

h2 {
  font-family: sans-serif;
  font-size: 1.25rem;
  margin: 1.5em 0.5em 0.5em;
}

#gradient {
  .menu {
    display: flex;
    background-color: #3cc57e;
  }
  
  .nav li {
    display: inline-flex;
  }
  
  .nav a {
    line-height: 50px;
    padding: 0 10px;
    color: white;
    text-decoration: none;
    font-family: Tahoma, sans-serif;
  }
  
  .nav li:hover {
    background-image: linear-gradient(to top, red 0 3px, transparent 3px 100%);
  }
}

#border {
  .menu {
    display: flex;
    background-color: #673ab7;
  }
  
  .nav li {
    display: inline-flex;
    border-top: 3px solid transparent;
    border-bottom: 3px solid transparent;
  }
  
  .nav a {
    line-height: 44px;
    padding: 0 10px;
    color: white;
    text-decoration: none;
    font-family: Tahoma, sans-serif;
  }
  
  .nav li:hover {
    border-bottom-color: red;
  }
}

#pseudo {
  .menu {
    display: flex;
    background-color: #00BCD4;
  }
  
  .nav li {
    display: inline-flex;
    position: relative;
  }
  
  .nav li::after {
    position: absolute;
    left: 0;
    bottom: 0;
    content: '';
    width: 100%;
    height: 0;
    background-color: red;
  }
  
  .nav a {
    line-height: 50px;
    padding: 0 10px;
    color: white;
    text-decoration: none;
    font-family: Tahoma, sans-serif;
  }
  
  .nav li:hover::after {
    height: 3px;
  }
}

#box-shadow {
  .menu {
    display: flex;
    background-color: #607D8B;
  }
  
  .nav li {
    display: inline-flex;
  }
  
  .nav a {
    line-height: 50px;
    padding: 0 10px;
    color: white;
    text-decoration: none;
    font-family: Tahoma, sans-serif;
  }
  
  .nav li:hover {
    box-shadow: inset 0 -3px 0 0 red;
  }
}