Yes, this includes animating the closing of the element too!

Introduction

Animating the <details> HTML element using only pure CSS can be a challenging task, as many articles, posts, and tutorials either rely on JavaScript or offer incomplete solutions that are unable to animate the closing of the element or produce a smooth animation.

However, with a bit of creativity, it is possible to create a fully-animated <details> element using pure CSS.

This solution reproduces the example from Chris Coyier's CSS Tricks post, but achieves the same smooth animation without using any JavaScript – only CSS – for both opening and closing the element.

The CSS Solution

The key is to start the page with the details element in its open state (<details open>), then change its max-height value by toggling a checkbox, and use a transition effect to animate that change.

details {
   max-height: 4rem; /* Set a max-height value just enough to show the summary */
   overflow: hidden; /* Hide the rest of the content */
   transition: max-height 400ms ease-out; /* Animate the change */
}

To control the change of the element's max-height value, you will need a checkbox and associate it with the <details> element. You can choose from two approaches to do this, and both work great:

1. Using the + adjacent sibling combinator
2. Using the :has() pseudo-class

input:checked + details,
details:has(input:checked) {
    max-height: 576px; /* Set a max-height value enough to show all the content */
}

In the CSS, the selector input:checked + details selects a <details> element that immediately follows a checked input element. On the other hand, the selector details:has(input:checked) selects a <details> element that has a checked input element as its descendant.

The HTML

The difference between the two approaches in HTML is that the checkbox input must come immediately before the <details> element in the first approach, and the checkbox must be a descendant of the <details> element in the second approach.

Inside the <summary> tag of the <details> element , place a <label> that toggles the checkbox:

<!-- approach #1 (checkbox comes immediately before the details element) -->
<input type="checkbox" name="approach-one" id="approach-one" />
<details open>
  <summary>
    <label for="approach-one">This one uses the adjacent sibling combinator (+) approach</label>
  </summary>

  <div class="rest-of-the-content">...</div>
</details>

<!-- approach #2 (checkbox is a descendant of the details element -->
<details open>
  <summary>
    <input type="checkbox" name="approach-two" id="approach-two" />
    <label for="approach-two">This one uses :has() pseudo-class approach</label>
  </summary>

  <div class="rest-of-the-content">...</div>
</details>

A note about Firefox browser

One caveat to consider when using the :has() approach is that in Firefox the user must explicitly enable this feature (see caniuse.com and MDN Web Docs).

Final result