JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <h1>
    The underline shouldn't extend to the right border.
  </h1>
</div>

<div class="container">
  <h1 class="correct-behavior">
    It should look like this instead.
  </h1>
</div>

CSS

.container {
  width: 300px;
  height: 200px;
  border: 1px solid black;
  margin-bottom: 20px;
}

h1 {
  display: inline;
  position: relative;
}

h1:after {
  position: absolute;
  bottom: 0; left: 0; right: 0;
  border-bottom: 3px solid black;
  content: '';
}