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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsFor a smooth expanding div, animate between explicit pixel heights—not from height: 0 to height: auto. Measure the content with JavaScript’s scrollHeight, use a real button to control the panel, and account for keyboard access, reduced motion, and content changes.
Why height: auto does not animate reliably
CSS transitions interpolate between values the browser can calculate. auto is not a fixed numeric endpoint, so a transition to or from it can be unpredictable. Set a numeric height for both ends of the transition instead. MDN’s guide to CSS transitions explains how transitions behave.
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Build a measured-height disclosure
This pattern measures the panel’s content, transitions to that pixel height when opening, and returns to zero when closing. It uses a native button and exposes the open state through aria-expanded.
<button type="button" aria-expanded="false" aria-controls="more-info">
Show more
</button>
<div id="more-info" class="more-content">
<p>Additional information goes here.</p>
</div>
<style>
.more-content {
height: 0;
overflow: hidden;
transition: height 250ms ease;
}
@media (prefers-reduced-motion: reduce) {
.more-content {
transition: none;
}
}
</style>
<script>
const button = document.querySelector('[aria-controls="more-info"]');
const panel = document.getElementById('more-info');
let isOpen = false;
button.addEventListener('click', () => {
isOpen = !isOpen;
button.setAttribute('aria-expanded', String(isOpen));
button.textContent = isOpen ? 'Show less' : 'Show more';
if (isOpen) {
panel.style.height = `${panel.scrollHeight}px`;
panel.addEventListener('transitionend', function onOpen(event) {
if (event.propertyName !== 'height') return;
panel.style.height = 'auto';
panel.removeEventListener('transitionend', onOpen);
});
} else {
panel.style.height = `${panel.scrollHeight}px`;
requestAnimationFrame(() => {
panel.style.height = '0px';
});
}
});
</script>The opening transition ends at a measured height; after it completes, the script changes the height to auto so the open panel can reflow as its contents change. The closing handler first sets the current content height, then schedules the transition to zero on the next animation frame. This avoids trying to animate directly from auto.
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Keep the accessible state in sync
- Use a
<button type="button">, which works with keyboard activation, rather than a clickable generic element. - Keep
aria-controlspointed at the panel’s unique ID and updatearia-expandedon every toggle. - Change the button label to “Show less” while the panel is open, then back to “Show more” when closed.
- Do not apply
hiddenordisplay: nonebefore the closing animation finishes. That removes the panel from the accessibility tree and prevents it from animating. MDN documents the accessibility effect ofdisplay: none.
The example assumes the panel is initially collapsed and has no competing script changing its height. If the content changes while open, the temporary auto height allows it to reflow. Before a later close, the handler measures the current content again. If you need to keep an explicit height while content changes, use a ResizeObserver to update it.
Choose between JavaScript height and CSS max-height
| Approach | Animation control | Dynamic content and clipping | Trade-off |
|---|---|---|---|
| Measured numeric height with JavaScript | Transitions to the panel’s measured height. | Can measure variable content; remove the temporary fixed height after opening so content can reflow. | More implementation and state management, but gives the most predictable height animation. |
CSS max-height |
Can transition between a collapsed value and a chosen maximum. | Can clip content if the maximum is too small. If the maximum is much larger than the content, the perceived timing may not match the visible expansion. | Simpler when there is a trustworthy upper bound on content height. |
A CSS-only max-height transition is reasonable for short, bounded content. For content of varying length, measuring the actual height avoids choosing an arbitrary ceiling. These trade-offs follow from animating numeric dimensions rather than relying on auto.
Use native details when custom animation is not necessary
<details> and <summary> provide built-in disclosure semantics and browser-managed open and closed states:
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<summary>Show more</summary>
<p>Additional information goes here.</p>
</details>The element can also be opened or closed by script, but it has no built-in transition between those states. Choose it when native disclosure behavior matters more than a custom expansion animation. MDN’s <details> reference describes its behavior.
Respect reduced motion and text resizing
The @media (prefers-reduced-motion: reduce) rule in the example removes the transition for users whose operating system requests less non-essential motion. The content and button states should remain the same when motion is disabled; only the animation changes. MDN explains the reduced-motion preference.
Test the panel with zoom and larger text, too. A fixed height that is adequate at the default text size may obscure content after text is enlarged. Using auto while the panel is open helps it adapt; do not leave a fixed expanded height in place unless you update it when content or layout changes. MDN’s height reference covers height values and their behavior.
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