<div>
  <details open>
    <summary>
      How do you create an accordion?
    </summary>
    <div>
      Easy! As long as you don't have to support IE11 or older browsers you could use <code>&lt;details&gt;</code> and <code>&lt;summary&gt;</code> natively.
    </div>
  </details>
  <details>
    <summary>
      What if I have to support IE11 or older browsers?
    </summary>
    <div>
      No worries. The fallback for these elements is quite good. They will display as open. You won't get the open/close mechanism, but you won't lose any content either.
    </div>
  </details>
  <details>
    <summary>
      What type of content can I have inside one of these?
    </summary>
    <div>
      Almost anything you'd like. The <code>&lt;details&gt;</code> element allows all <a href="https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Content_categories#flow_content" target="_blank">flow content</a>, which is basically everything.
    </div>
  </details>
  <details>
    <summary>
      How does it work?
    </summary>
    <div>
      The <code>&lt;details&gt;</code> element encapsulates the <code>&lt;summary&gt;</code> element. The <code>&lt;summary&gt;</code> becomes the 'label' for the <code>&lt;details&gt;</code> and acts like a button. When clicked, the attribute <code>open</code> is added to the <code>&lt;details&gt;</code> element, making it display. You can therefore style the open and closed states seperately if you'd like.
    </div>
  </details>
</div>

@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800;900&display=swap');

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

:root {
  font-size: 16px;
}

body {
  font-family: "Inter", sans-serif;
  line-height: 1.5;
  min-height: 100vh;
  font-size: 1.25rem;
}

*:focus {
  outline: none;
}

body > div {
  width: 90%;
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
  margin-top: 5rem;
  margin-bottom: 5rem;
}

details div {
  border-left: 2px solid #000;
  border-right: 2px solid #000;
  border-bottom: 2px solid #000;
  padding: 1.5em;
}

details div > * + * {
  margin-top: 1.5em;
}

details + details {
  margin-top: .5rem;
}

summary {
  list-style: none;
}

summary::-webkit-details-marker {
  display: none;
}

summary {
  border: 2px solid #000;
  padding: .75em 1em;
  cursor: pointer;
  position: relative;
  padding-left: calc(1.75rem + .75rem + .75rem);
}

summary:before {
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
  left: .75rem;
  content: "↓";
  width: 1.75rem;
  height: 1.75rem;
  background-color: #000;
  color: #FFF;
  display: inline-flex;
  justify-content: center;
  align-items: center;
  flex-shrink: 0;
}

details[open] summary {
  background-color: #eee;
}

details[open] summary:before {
  content: "↑";
}

summary:hover {
  background-color: #eee;
}

a {
  color: inherit;
  font-weight: 600;
  text-decoration: none;
  box-shadow: 0 1px 0 0;
}

a:hover {
  box-shadow: 0 3px 0 0;
}

code {
  font-family: monospace;
  font-weight: 600;
}