<div class="app" data-attr="without something() selector">
  <a href="">No has underline</a>
  <nav>
    <a href="">no has underline</a>
  </nav>
</div>

<div class="app" data-attr="whit something() selector">
  <a href="">Has underline with :something selector</a>
  <nav>
    <a href="">Has underline with :something selector</a>
  </nav>
</div>

body {
  width: 100vw;
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: flex-start;
  padding: 2vw;
  background: #eaeaea;
}

a {
  font-size: 1.3rem;
  display: block;
  margin: 10px;
}

.app {
  border-radius: 8px;
  padding:30px 20px 20px;
  background: #fff;
  border: 1px solid #aeaeae;
  margin: 10px;
  position: relative;
  min-width: 80vh;
  
  &::before {
    content: attr(data-attr);
    position: absolute;
    padding: 10px;
    background: linear-gradient(to right, #f36, #f63);
    color: #fff;
    border-radius: 5px;
    top: -25px;
    white-space: nowrap;
  }
}

.app[data-attr~="without"] a:not(:hover) {
  text-decoration: none;
  color: red;
}

.app[data-attr~="without"] nav a{
//   Has no effect
  text-decoration: underline;
  color: green;
}

.app[data-attr~="whit"] a:something(:not(:hover)) {
    text-decoration: none;
  color: red;
}

.app[data-attr~="whit"] nav a {
  // Has  effect
  text-decoration: underline;
  color: green;
}