<div class="container">
  <div class="hover-container">
    <h1>Underline Link Effect</h1>
    <p><small>(hover over above text please)</small></p>
  </div>
  
  <p class="body-text">I made this little pen because I kept forgetting how to achieve this effect. </p>
  <p class="body-text"><strong>Note:</strong> Edit <code>transform-origin</code> to change how where the animation starts. It's currently set to <code>left</code>.<br /> <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/transform-origin">Read more about this property on MDN.</a> </p>
</div>

/* the code you want is around line 45  */

@import url("https://fonts.googleapis.com/css?family=Montserrat:400,700");

* {
  padding: 0;
  margin: 0;
}

html {
  box-sizing: border-box;
}
*,
*:before,
*:after {
  box-sizing: inherit;
}

body {
  font-family: Montserrat, sans-serif;
  height: 100%;
  min-height: 100vh;
  background-color: #ebf3ff;
}

.container {
  display: flex;
  flex-flow: column;
  width: 60%;
  height: 100vh;
  margin: 0 auto;
  align-items: center;
  justify-content: center;
}

.hover-container {
  text-align: center;
  margin-bottom: 2em;
}

.body-text {
  margin: 0.5em 0;
}

/* --- this is the code you probably want ---- */

h1 {
  border-bottom: 3px solid #ff8cbc;
  font-size: 3em;
  transition: all 0.25s linear;
  position: relative;
}

h1:before {
  content: "";
  display: block;
  width: 100%;
  height: 3px;
  background-color: #61a3ff;
  position: absolute;
  left: 0;
  bottom: -3px; /* this is to match where the border is */
  transform-origin: left; 
  transform: scale(0);
  transition: 0.25s linear;
/*   will-change: transform; */
}

h1:hover:before {
  transform: scale(1);
}