<div>
  <h2 class="all">20 CSS Tips That Greatly Improve User Experience</h2>
  <h3 class="all">bytefish</h3>
  <div>tel num: <span class="all">12345678</span></div>
  <p class="all">Lorem ipsum dolor sit amet consectetur adipisicing elit. Sequi velit eos asperiores labore voluptate atque doloremque ad assumenda nihil quia ullam ipsum in non magni quam facere fugiat, ipsam amet.</p>
</div>

.all{
  user-select: all;
}

.all::selection{
  background-color: yellow;
  color: red;
}