::after {
  /* ... */
}

<p class="boring-text">Here is some plain old boring text.</p>
<p>Here is some normal text that is neither boring nor exciting.</p>
<p class="exciting-text">Contributing to MDN is easy and fun.</p>

.exciting-text::after {
  content: " <- EXCITING!";
  color: green;
}

.boring-text::after {
  content: " <- BORING";
  color: red;
}

<span class="ribbon">Look at the orange box after this text. </span>

.ribbon {
  background-color: #5bc8f7;
}

.ribbon::after {
  content: "This is a fancy orange box.";
  background-color: #ffba10;
  border-color: black;
  border-style: dotted;
}

<p>
  Here we have some
  <span tabindex="0" data-descr="collection of words and punctuation">
    text
  </span>
  with a few
  <span tabindex="0" data-descr="small popups that appear when hovering">
    tooltips</span
  >.
</p>

span[data-descr] {
  position: relative;
  text-decoration: underline;
  color: #00f;
  cursor: help;
}

span[data-descr]:hover::after,
span[data-descr]:focus::after {
  content: attr(data-descr);
  position: absolute;
  left: 0;
  top: 24px;
  min-width: 200px;
  border: 1px #aaaaaa solid;
  border-radius: 10px;
  background-color: #ffffcc;
  padding: 12px;
  color: #000000;
  font-size: 14px;
  z-index: 1;
}