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Animating width can trigger repeated layout work because it changes an element’s geometry; in a JSlet benchmark, a width animation produced 273.4 ms of layout work over 1.8 seconds. That figure describes one specific test—not a universal cost. Whether width is the right property depends on whether surrounding content must actually reflow and what profiling shows on your page.
What the 273.4 ms benchmark measured
JSlet reported that a test animating the width of 3,000 inline-block elements produced 110 layout passes and 273.4 ms of layout work over 1.8 seconds. The same briefing reported 213.6 ms of paint work for the width run. The test used Chrome 136 headless on Windows 11, and summed event durations from Chrome DevTools Protocol tracing. These are the publisher’s reported measurements; the exact result was not independently replicated in the available sources. JSlet’s benchmark briefing
In the comparison run, animating with transform produced one initial layout pass, with about 0.01 ms of layout per frame, according to the same briefing. This does not mean every transform animation has zero rendering cost, or that width animation always takes 273.4 ms. Browser, device, page structure, and measurement conditions all affect the result.
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Why changing width can cost more
Changing width changes an element’s geometry. The browser may need to recalculate layout for that element and other affected content, then paint the result. On a page with many elements, this work can add up. web.dev’s animation performance guidance
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A transform can often move or visually scale an element without making neighboring content reflow as it would when the element’s actual width changes. That difference is useful only when it produces the intended interface: if adjacent content is supposed to move as the width changes, replacing the width animation with a transform may alter the behavior rather than optimize an equivalent effect.
Transforms and opacity are often more readily handled by the compositor, but CSS alone does not guarantee a faster animation. Layout or paint work can require main-thread work for both CSS and JavaScript animations. web.dev’s overview of animations
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Choose the property that matches the behavior
| Question | Animating width | Animating transform |
|---|---|---|
| Should neighboring content reflow? | Can change layout and move or resize affected elements. | Can visually move or scale an element without the same surrounding reflow. |
| What should you expect to profile? | Check for layout and paint activity during the animation. | Check actual rendering and frame behavior; compositor-friendly does not mean cost-free. |
| When may it fit? | When the interface needs a real geometry change. | When the visual effect can be achieved without changing surrounding layout. |
These are rendering distinctions, not a rule that one property is always better. Compare the implementations against the desired layout behavior and measured performance on the target page. web.dev’s animation performance guidance
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- Open the page in Chrome and start a recording in the Performance panel while the animation runs. Inspect rendering activity, including layout and paint, instead of judging performance from the property name alone. Chrome DevTools Performance documentation
- For supported CSS animations, transitions, Web Animations, and View Transitions, use the Animations panel to capture and inspect effects. The panel does not support
requestAnimationFrame-based animations; record those in the Performance panel instead. Chrome DevTools Animations documentation - Compare the original and changed versions under the same browser, device, page state, and interaction conditions. Check both rendering activity and whether the visual result and layout behavior still meet the design requirement.
- Repeat the comparison on the devices and browsers that matter for your users. A result from one headless Chrome setup cannot establish how every target device will behave.
Does the same advice apply to transitions and JavaScript animations?
Yes: the important factor is the property being changed and the rendering work that change causes, not whether the effect is called a transition or an animation. The same principle applies to the Web Animations API, including element.animate(). Profile the actual effect in its page context rather than assuming a particular API or CSS syntax makes it inexpensive. web.dev’s overview of animations
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Use will-change sparingly
will-change is a hint to the browser, not a blanket fix for expensive animations. Applying it indiscriminately can waste resources and worsen performance. Use it only where a measured, specific need justifies it. web.dev’s animation guide
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