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HTML

              
                <details>
  <summary>Click to expand this details with a WAAPI sliding effect</summary>
  <div class="content">
    <p>
      Lorem, ipsum dolor sit amet consectetur adipisicing elit. Modi unde, ex rem voluptates autem aliquid veniam quis temporibus repudiandae illo, nostrum, pariatur quae! At animi modi dignissimos corrupti placeat voluptatum!
    </p>
    <img src="https://placebear.com/400/200" alt="">
    <p>
      Facilis ducimus iure officia quos possimus quaerat iusto, quas, laboriosam sapiente autem ab assumenda eligendi voluptatum nisi eius cumque, tempore reprehenderit optio placeat praesentium non sint repellendus consequuntur? Nihil, soluta.
    </p>
  </div>
</details>
<details>
  <summary>Click to expand this details with a WAAPI sliding effect</summary>
  <div class="content">
    <p>
      Lorem, ipsum dolor sit amet consectetur adipisicing elit. Modi unde, ex rem voluptates autem aliquid veniam quis temporibus repudiandae illo, nostrum, pariatur quae! At animi modi dignissimos corrupti placeat voluptatum!
    </p>
    <img src="https://placebear.com/400/200" alt="">
    <p>
      Facilis ducimus iure officia quos possimus quaerat iusto, quas, laboriosam sapiente autem ab assumenda eligendi voluptatum nisi eius cumque, tempore reprehenderit optio placeat praesentium non sint repellendus consequuntur? Nihil, soluta.
    </p>
  </div>
</details>
<details>
  <summary>Click to expand this details with a WAAPI sliding effect</summary>
  <div class="content">
    <p>
      Lorem, ipsum dolor sit amet consectetur adipisicing elit. Modi unde, ex rem voluptates autem aliquid veniam quis temporibus repudiandae illo, nostrum, pariatur quae! At animi modi dignissimos corrupti placeat voluptatum!
    </p>
    <img src="https://placebear.com/400/200" alt="">
    <p>
      Facilis ducimus iure officia quos possimus quaerat iusto, quas, laboriosam sapiente autem ab assumenda eligendi voluptatum nisi eius cumque, tempore reprehenderit optio placeat praesentium non sint repellendus consequuntur? Nihil, soluta.
    </p>
  </div>
</details>
              
            
!

CSS

              
                body {
  background: white;
  font-family: 'Open Sans', sans-serif;
}
summary {
  background: #8dd792;
  padding: 5px;
  border: 2px solid transparent;
  outline: none;
  &:focus {
    border-color: black;
  }
}

details {
  max-width: 500px;
  box-sizing: border-box;
  margin-top: 5px;
}
.content {
  border-top: none;
  padding: 10px;
  border: 2px solid #8dd792;
  border-top: none;
}

p {
  margin: 0;
  padding-bottom: 10px;
  &:last-child {
    padding: 0;
  }
}
img {
  max-width: 100%;
}
              
            
!

JS

              
                class Accordion {
  constructor(el) {
    // Store the <details> element
    this.el = el;
    // Store the <summary> element
    this.summary = el.querySelector('summary');
    // Store the <div class="content"> element
    this.content = el.querySelector('.content');

    // Store the animation object (so we can cancel it if needed)
    this.animation = null;
    // Store if the element is closing
    this.isClosing = false;
    // Store if the element is expanding
    this.isExpanding = false;
    // Detect user clicks on the summary element
    this.summary.addEventListener('click', (e) => this.onClick(e));
  }

  onClick(e) {
    // Stop default behaviour from the browser
    e.preventDefault();
    // Add an overflow on the <details> to avoid content overflowing
    this.el.style.overflow = 'hidden';
    // Check if the element is being closed or is already closed
    if (this.isClosing || !this.el.open) {
      this.open();
    // Check if the element is being openned or is already open
    } else if (this.isExpanding || this.el.open) {
      this.shrink();
    }
  }

  shrink() {
    // Set the element as "being closed"
    this.isClosing = true;
    
    // Store the current height of the element
    const startHeight = `${this.el.offsetHeight}px`;
    // Calculate the height of the summary
    const endHeight = `${this.summary.offsetHeight}px`;
    
    // If there is already an animation running
    if (this.animation) {
      // Cancel the current animation
      this.animation.cancel();
    }
    
    // Start a WAAPI animation
    this.animation = this.el.animate({
      // Set the keyframes from the startHeight to endHeight
      height: [startHeight, endHeight]
    }, {
      duration: 400,
      easing: 'ease-out'
    });
    
    // When the animation is complete, call onAnimationFinish()
    this.animation.onfinish = () => this.onAnimationFinish(false);
    // If the animation is cancelled, isClosing variable is set to false
    this.animation.oncancel = () => this.isClosing = false;
  }

  open() {
    // Apply a fixed height on the element
    this.el.style.height = `${this.el.offsetHeight}px`;
    // Force the [open] attribute on the details element
    this.el.open = true;
    // Wait for the next frame to call the expand function
    window.requestAnimationFrame(() => this.expand());
  }

  expand() {
    // Set the element as "being expanding"
    this.isExpanding = true;
    // Get the current fixed height of the element
    const startHeight = `${this.el.offsetHeight}px`;
    // Calculate the open height of the element (summary height + content height)
    const endHeight = `${this.summary.offsetHeight + this.content.offsetHeight}px`;
    
    // If there is already an animation running
    if (this.animation) {
      // Cancel the current animation
      this.animation.cancel();
    }
    
    // Start a WAAPI animation
    this.animation = this.el.animate({
      // Set the keyframes from the startHeight to endHeight
      height: [startHeight, endHeight]
    }, {
      duration: 400,
      easing: 'ease-out'
    });
    // When the animation is complete, call onAnimationFinish()
    this.animation.onfinish = () => this.onAnimationFinish(true);
    // If the animation is cancelled, isExpanding variable is set to false
    this.animation.oncancel = () => this.isExpanding = false;
  }

  onAnimationFinish(open) {
    // Set the open attribute based on the parameter
    this.el.open = open;
    // Clear the stored animation
    this.animation = null;
    // Reset isClosing & isExpanding
    this.isClosing = false;
    this.isExpanding = false;
    // Remove the overflow hidden and the fixed height
    this.el.style.height = this.el.style.overflow = '';
  }
}

document.querySelectorAll('details').forEach((el) => {
  new Accordion(el);
});

              
            
!
999px

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