<button class="reveal-button">Click me</button>
<section class="hidden-section">Subway tile you probably haven't heard of them offal keffiyeh. Sartorial irony meh cold-pressed viral heirloom thundercats. Woke tumblr taiyaki lyft neutra. Crucifix hot chicken brunch you probably haven't heard of them, coloring book retro health goth master cleanse hell of vexillologist.</section>
html {
  text-align: center;
  padding-top: 1em;
  background: #f6ddda;
}

button {
  padding: 1em 1.2em;
  background: #eb6ba0;
  color: white;
  font-size: 20px;
  border-radius: .3em;
  border: none;
  border-bottom: 4px solid #aa4b72;
  cursor: pointer;
}

section {
  max-width: 700px;
  margin: 2em auto;
  font-size: 1.2em;
  line-height: 1.5;
  color: #2b2c3b;
  text-align: left;
}

.hidden-section {
    display: none;
}
.hidden-section.reveal {
    display: block;
}
/* Selecting DOM elements */
const revealButton = document.querySelector(".reveal-button");
const hiddenSection = document.querySelector(".hidden-section");

/* Callback function */
function revealSection() {
    if (hiddenSection.classList.contains("reveal")) {
        hiddenSection.classList.remove("reveal");
    } else {
        hiddenSection.classList.add("reveal");
    }
}

/* Event listener */
revealButton.addEventListener("click", revealSection, false);

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.