<div id="demo">
  <!-- Tab 1 -->
  <input type="radio" name="demo-tabs" id="tab1" checked>
  <label for="tab1">
    Label 1
  </label>
  <section>
    Content 1
  </section>
  
  <!-- Tab 2 -->
  <input type="radio" name="demo-tabs" id="tab2">
  <label for="tab2">
    Label 2
  </label>
  <section>
    Content 2
  </section>
  
</div>

body {  
  font-size: 1em;  
}
#demo > * {
  padding: 1em;
  border: 1px solid #333;
  display: inline-block;
}

#demo > input:checked + label {
  border-width: 5px;
  background: yellow;
}
#demo > section {
  opacity: 0.3;
}
#demo > input:checked + label + section {
  opacity: 1;
  border-width: 5px;  
}