<section class="accordion">
  <details open>
    <summary>CSS-only Accordion</summary>
    <p>Using the &lt;details&gt; element</p>
    <p>And no unnecessary DOM</p>
  </details>
  <details>
    <summary>Animated open-close</summary>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla mollis consectetur iaculis. Fusce vestibulum pulvinar est. Fusce faucibus aliquet purus hendrerit efficitur. Phasellus ante urna, mattis et ligula eu, venenatis vulputate ex. Fusce quis enim aliquam, finibus mi vel, sagittis urna. Morbi in enim placerat, sagittis ante vitae, ultrices nisl. Duis molestie quis ante vitae rhoncus. Integer ultricies libero ut efficitur sagittis. Vestibulum euismod pharetra ligula, eget bibendum turpis gravida non. Curabitur pulvinar nulla id porttitor vehicula. Vivamus efficitur est sed lectus commodo, in facilisis magna vehicula. Sed in massa nunc. Praesent sodales neque nec lectus auctor, id faucibus libero malesuada. Fusce tempus, diam quis consequat efficitur, erat justo pretium massa, et vehicula risus lectus non quam. Nam faucibus luctus metus vitae malesuada.</p>
  </details>
  <details>
    <summary>Even put images in them</summary>
    <img src="https://images.unsplash.com/photo-1513006666308-523b5f33c3dd?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=2592&q=80" />
  </details>
  <details>
    <summary>Woops, no content!</summary>
  </details>
</section>

details > summary:focus {
  outline: none;
}
details > summary::-webkit-details-marker {
  display: none;
}
details > summary {
  list-style: none;
  border: solid 1px teal;
  border-radius: 0.5rem;
}
details[open] > summary {
  border-bottom-left-radius: 0;
  border-bottom-right-radius: 0;
}
details[open] > :not(summary) {
  border-left: solid 1px teal;
  border-right: solid 1px teal;
}
details[open] > :last-child {
  border-bottom: solid 1px teal;
  border-bottom-left-radius: 0.5rem;
  border-bottom-right-radius: 0.5rem;
}
details > summary::before {
  content: "";
  display: inline-block;
  width: 0.5rem;
  height: 0.5rem;
  border: solid 2px black;
  border-left-color: transparent;
  border-bottom-color: transparent;
  margin-right: 0.6rem;
  transition: transform 0.2s;
  transform: rotate(-45deg) translate(-20%, 20%);
}
details[open] > summary::before {
  transform: rotate(135deg) translate(-20%, 20%);
}

/* layout */
section.accordion {
  max-width: 400px;
  width: 100%;
  padding: 10px; 
}
details {
  padding: 10px;
}
details > * {
  width: 100%;
  padding: 10px;
  margin: 0px;
}