Progressive enhancement

To begin, letʼs start with some essential HTML:

<details class="xx-accordion">
  <summary class="xx-accordion-summary">Always visible</summary>
  <div class="xx-accordion-content">
    <p>Hidden content</p>
    <p>More hidden content</p>
  </div>
</details>

You can see an example page here.

No doubt you have spotted the class attributes. We prefer to apply CSS by class. We can fall back to class-and-tag-name if we need more specificity, as youʼll see below.

So whatʼs up with the “xx-” prefix?

Simple. To avoid name clashes with imported CSS, we namespace our classes. For example, the Craft Code site might use “cc-” as in cc-accordion.

Letʼs add a bit of CSS to give it that “accordion” look and feel. Here we go:

.xx-accordion {
  background-color: #fafafa;
  border-radius: 5px;
  border: 1px solid #747481;
  color: #2c2c30;
}

.xx-accordion-summary {
  background-color: #0d4872;
  color: #fafafa;
  font-size: 1.25rem;
  margin: 0;
  padding: 1rem;
}

.xx-accordion-content {
  border-top: 1px solid #747481;
  padding: 0 1rem;
}

Again, we provide an example page.

This is already a workable accordion component. But most accordions have more than one expandable element. So letʼs add a few more elements:

<div class="xx-accordion-group">
  <details class="xx-accordion">
    <summary class="xx-accordion-summary">Katherine Johnson</summary>
    <div class="xx-accordion-content">
      <p>
        <a
          href="https://en.wikipedia.org/wiki/Katherine_Johnson"
          rel="external"
        >Creola Katherine Johnson</a> was an American mathematician
        whose calculations of orbital mechanics as a NASA employee
        were critical to the success of the first and subsequent
        U.S. crewed spaceflights.
      </p>
    </div>
  </details>
  <details class="xx-accordion">
    <summary class="xx-accordion-summary">Dorothy Vaughan</summary>
    <div class="xx-accordion-content">
      <p>
        <a
          href="https://en.wikipedia.org/wiki/Dorothy_Vaughan"
          rel="external"
        >Dorothy Jean Johnson Vaughan</a> was an American
        mathematician  and human computer who worked for the
        National Advisory Committee for Aeronautics (NACA), and
        NASA, at Langley Research Center in Hampton, Virginia.
      </p>
    </div>
  </details>
  <details class="xx-accordion">
    <summary class="xx-accordion-summary">Mary Jackson</summary>
    <div class="xx-accordion-content">
      <p>
        <a
          href="https://en.wikipedia.org/wiki/Mary_Jackson_(engineer)"
          rel="external"
        >Mary Jackson</a> was an American mathematician and
        aerospace engineer at the National Advisory Committee
        for Aeronautics (NACA), which in 1958 was succeeded by
        the National Aeronautics and Space Administration (NASA).
      </p>
    </div>
  </details>
</div>

We have also adjusted the CSS a bit to make it work with multiple elements:

.xx-accordion-group {
  border-radius: 5px;
  border: 1px solid #0d4872;
  padding: 0;
}

.xx-accordion {
  background-color: #fafafa;
  color: #2c2c30;
}

.xx-accordion-summary {
  background-color: #0d4872;
  border-bottom: 1px solid #fafafa;
  color: #fafafa;
  font-size: 1.25rem;
  margin: 0;
  padding: 1rem;
}

.xx-accordion:last-child summary {
  border-bottom: none;
}

.xx-accordion-content {
  padding: 0 1rem;
}

• We added the .xx-accordion-group properties. And moved the outer border from the individual elements to the group. We also changed the border color to match the <summary> background color.
• We removed the top border from the content block. Then we replaced it with a bottom border on the <summary> element. We also changed the color to the background color of the <details> elements. This makes it appear as a horizontal rule between the accordion elements.
• We donʼt want that bottom border on the last element in the accordion group. So we used .xx-accordion:last-child summary to remove that one bottom border.

Here is the full accordion example.

Rats! The accordion elements open and close instantly when you click on the summary. It would be nice if we animated the open and close.

We bet that your first thought will be that we can do this with CSS animations. We thought so, too. But it turns out that it is fiendishly difficult to make that work. By that we mean that we couldnʼt. Sigh.

CSS should always be your first choice for effects, but when CSS fails you, JavaScript steps up. We can do it the way we used to.

CSS transitions for the loss

Naturally, the first thing we tried was a CSS transition on the height of the content block. But this only works if you know the height of the block in advance. It wonʼt work (for us, anyway) with height set to auto or fit-content.

OK, fine! Instead we set the open height to a CSS property with a fallback to auto: var(--xx-accordion-height, auto). That oughta do it.

Now we need to set that property for each accordion element separately. Enter JavaScript!

JavaScript to the rescue

The first thing we want to do is write a function that grabs any and all accordion elements on the page. Then weʼll add an event listener to each of them. And we want our function to run once as soon as the DOM has finished loading.

Here, then, is our first pass at the JavaScript:

function enhanceAccordions () {
  const nodes = document.querySelectorAll(".xx-accordion-content")

  for (let node of nodes) {
    node.parentNode.open = true
    node.style = `--xx-accordion-height: ${node.clientHeight}px`
    node.parentNode.open = false
  }
}

globalThis.addEventListener("DOMContentLoaded", enhanceAccordions)

And our modified CSS:

.xx-accordion-content {
  height: 0;
  overflow-y: hidden;
  padding: 0 1rem;
  transition: height 0.5s ease-in-out;
}

.xx-accordion[open] .xx-accordion-content {
  height: var(--xx-accordion-height, auto);
  transition: height 0.5s ease-in-out;
}

And it does work. Beautifully … but intermittently. Try it. And then it stops working at all. And it never animates the close. Why not?

Because when the <details> element toggles closed, it sets its “open” value to false. And thatʼs that.

Slam!

OK, fine. Time to show it whoʼs boss.

Kicking butt and taking names

First, letʼs delete the CSS we added. Forget using a transition! Weʼll do this the old-fashioned way.

First issue: when the <details> element toggles shut, it closes the accordion abruptly. We need to prevent that. To do so, we will seize control.

Instead of handling the toggle event, letʼs capture the click on the <summary> element. Then we will prevent the click from bubbling up.

We still need to know the height of the open accordion, but we donʼt need it in our CSS. So letʼs use a property instead. Weʼll call it xxOpenHeight. (Do remember to replace xx with your own namespace.)

function toggleAccordion (event) {
  console.log(event.target.xxOpenHeight)
}

function enhanceAccordions () {
  const nodes = document.querySelectorAll(".xx-accordion")

  for (let node of nodes) {
    const summary = node.querySelector("summary")
    const content = node.querySelector(
      ".xx-accordion-content",
    )

    if (content) {
      node.open = true
      summary.xxOpenHeight = content.clientHeight
      node.open = false
    }

    summary?.addEventListener("click", toggleAccordion)
  }
}

globalThis.addEventListener("DOMContentLoaded", enhanceAccordions)

Check out the enhanced example and open the browser console. Note how clicking on the a summary prints the height of that accordionʼs content block to the console. Weʼre off to a good start.

Now letʼs complete the toggleAccordion function to animate the accordion elements. Weʼll explain this code line by line below.

function toggleAccordion (event) {
  event.preventDefault()

  const summary = event.target
  const accordion = summary.closest("details")
  const content = accordion.querySelector(".xx-accordion-content")
  const openHeight = summary.xxOpenHeight

  if (accordion.open) {
    content.style.maxHeight = `${openHeight}px`

    function shutAccordion() {
      const maxHeight = parseInt(content.style.maxHeight, 10)

      if (maxHeight > 0) {
        content.style.maxHeight = maxHeight < 25
          ? "0"
          : `${maxHeight - 25}px`

        setTimeout(shutAccordion, 10)

        return
      }

      accordion.open = false
    }

    shutAccordion()

    return
  }

  content.style.maxHeight = "0px"

  function openAccordion() {
    const maxHeight = parseInt(content.style.maxHeight, 10)

    accordion.open = true

    if (maxHeight < openHeight) {
      content.style.maxHeight = `${maxHeight + 15}px`

      setTimeout(openAccordion, 10)
    }
  }

  openAccordion()
}

The final version is here.

1. We need to prevent the element from toggling. We can do this with event.preventDefault() on line #2 above. Now we are responsible for opening and closing the accordion ourselves.
2. Next, we use the event.target to get the <details> (accordion) element, our content block, and the xxOpenHeight. Lines #4 to #7.
3. Now we have two possibilities. Either the accordion is open and we want to close it, or vice versa. So we check if the accordion is open on line #9.
4. We need the maxHeight property of the content blockʼs style attribute. We will use it to control the height of the block. So if the accordion is open, then we must make sure that maxHeight it is set to xxOpenHeight. See line #10.
5. Now we will use setTimeout recursively to reduce the maxHeight bit by bit. Weʼll call it every few milliseconds until we have shut the accordion. We create our shutAccordion function on lines #12 to #26.
6. Hmm … maxHeight is a string. Urk. So we will convert it to a number with parseInt to decrement it. Then weʼll recreate the string. We get the integer on line #13. If our maxHeight is still greater than zero, we subtract 25 from it on lines #16 to #18. Then we call our shutAccordion function recursively. We set the timeout to 10 milliseconds. Thatʼs a nice frame rate.
7. When the maxHeight reaches zero, we set accordion.open to false (line #25) and weʼre good to go.
8. Now that weʼve defined our shutAccordion recursive function, we need to call it to start the recursion (animation). We do so on line #28.
9. But what if the accordion is closed? Then we set the maxHeight to zero on line #33 to be sure. This also covers the initial case.
10. We define our openAccordion recursive function on lines #35 to #45.
11. Again, we begin by parsing the height from maxHeight on line #36. And we immediately set the accordion to “open” on line #38. This has no visual effect yet because the height of the content block is currently zero.
12. Now, on lines #40 to #44, if the maxHeight is still less than the measured openHeight, we add 15 pixels to the maxHeight. Then we call the function again using setTimeout to delay for 10 milliseconds. When the content block is fully open, this conditional will fail and the recursion will stop.
13. And, as with the shutAccordion function above, we have to call the function to start the animation. We do this on line #47.

Not that difficult!

Try the final example above. You might note an issue with the content of the content block becoming visible instantly. This happens even though the height is zero.

That is because we havenʼt hidden the overflow. We can do that by adding overflow-y: hidden to the .sb-accordion-content CSS.

Once we do that, then our accordion should animate flawlessly. Now disable JavaScript in the DevTools of your browser. Check it out. It still works. It just doesnʼt animate. But we can live with that, right?

And if you disable both CSS and JavaScript, then it still works fine! It even works on the Lynx text-only browser, which doesnʼt recognize the <details> element. But that just means that the accordion is always open.

Again, we can live with that.

Thatʼs progressive enhancement for you. Our code is simple and responsive and works for everyone. And if you have CSS and JS enabled, then you get a nicer experience.