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HTML 

              
                <main class="page-layout">
  <section class="faq">
    <div>
      <span class="section-icon" aria-hidden='true'>🤔</span>
      <h2 class="section-title">Frequently asked questions</h2>
    </div>
    <div class="questions-and-answers">

      <details>
        <summary>Can I animate details/summary elements?</summary>
        <p>This used to be something that was impossible, but now thanks to modern CSS, it's actually pretty easy to do!</p>
      </details>

      <details>
        <summary>How is it possible?</summary>
        <p>There are a few different ways that you can do it. One of them is using <code>calc-size()</code>, which has some extra things you could do with it, but right now the easiest way is to use a combination of <code>interpolate-size: allow-keywords;</code> and <code>transition-behavior: allow-discrete</code>.</p>
      </details>

      <details>
        <summary>What about browser support?</summary>
        <p>Browser support isn't great as of the time of creating this demo, but it'll only get better from here. Also, it is a great <em>progressive enhancement</em>. This means that it won't break anything for people on older browsers, they just won't get the fancy animation you've created, while people on newer browser will get it!</p>
      </details>

      <details>
        <summary>What's the catch?</summary>
        <p>There is none! It's easy to do and it works great! I feel like that's too short, and I want to use this demo to show the animations and need a longer box to show it properly so here's a little bit of extra filler text lorem ipsum dolor sit amet consectetur adipisicing elit. Modi at ipsum dolor tempore impedit corrupti ad fugiat voluptate reiciendis voluptatum!</p>
      </details>

    </div>
  </section>
</main>
!

CSS 

              
                :root {
  interpolate-size: allow-keywords;
}

details {
  background: hsl(0 0% 5%);
  border: 1px solid hsl(0 0% 12%);
  padding: 0.5rem 1rem;
  border-radius: 0.25rem;

  overflow: hidden;
}

/* don't add margin/padding to this, add it to the actual
   stuff inside the details content */
details::details-content {
  color: hsl(0 0% 78%);
  font-size: 1rem;
  block-size: 0;
  transition: block-size 1s, content-visibility 1s;
  transition-behavior: allow-discrete;
}

details:open::details-content {
  block-size: auto;
}

@layer general-styling {
  html {
    font-family: system-ui;
    color-scheme: dark;
  }

  body {
    margin: 0;
    font-size: 1.5rem;
    line-height: 1.6;
    min-height: 100dvh;

    background-image: radial-gradient(
      in oklch circle at top left,
      hsl(303, 100%, 26%, 0.08),
      transparent
    );
    background-repeat: no-repeat;
  }

  h1,
  h2,
  h3,
  h4,
  p,
  details,
  summary {
    margin-block-end: 0;
  }

  .page-layout {
    display: grid;
    grid-template-columns:
      minmax(2rem, 1fr)
      min(960px, 100% - 4rem)
      minmax(2rem, 1fr);

    & > * {
      grid-column: 2 / 3;
    }

    margin-block: 5rem;
    container-type: inline-size;
    container-name: page-layout;
  }

  .section-title {
    font-size: 2rem;
    margin: 0;
    line-height: 1;
  }

  .section-icon {
    font-size: 3rem;
  }

  .faq {
    display: grid;
    gap: clamp(1rem, 10vw, 2.5rem);

    @container (width > 650px) {
      grid-template-columns: 250px 1fr;
    }
  }

  .questions-and-answers {
    margin-block-start: 3.5rem;
  }

  .questions-and-answers {
    display: grid;
    gap: 1rem;
  }

  details * {
    margin: 0;
  }

  details > p {
    padding-block: 1rem;
  }

  summary {
    color: hsl(0 0% 98%);
    line-height: 1.3;
    margin-inline-start: 1rem;
    list-style-position: outside;
  }

  summary::marker {
    font-size: 0.75em;
    color: hsl(0 0% 40%);
  }
}
!

JS 

              
                const navToggle = document.querySelector(
  '[aria-controls="primary-navigation"]'
);

navToggle.addEventListener("click", () => {
  const navOpened = navToggle.getAttribute("aria-expanded");

  if (navOpened === "false") {
    navToggle.setAttribute("aria-expanded", true);
  } else {
    navToggle.setAttribute("aria-expanded", false);
  }
});
!
999px

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