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Some CSS updates change an element’s geometry, so the browser may need to recalculate layout and redraw affected pixels. Others change only how existing pixels look, or can be handled during composition without repainting. That difference explains why changing width is often more expensive than changing color, and why transform and opacity are common choices for animation—but not automatic performance guarantees.
What happens when a browser updates the screen?
A useful rendering model has four stages: style recalculation, layout, paint, and composition. A change may require only some of them, and work in one stage can cause later stages to run too.
- Style recalculation: The browser determines the computed styles that apply to elements.
- Layout: It calculates the size and position of rendered boxes. This stage is also commonly called reflow when layout is recalculated after a change.
- Paint: It draws visual details—such as text, colors, borders, shadows, and images—into pixels.
- Composition: The browser combines the painted layers into the displayed image. Some updates to a suitable independent layer can be handled here without repainting that layer.
This is a mental model, not a promise that every browser handles every property or page in exactly the same way. Layout changes can lead to paint and composition work; a paint change can require composition afterward. How much of the page is affected depends on the page and the browser’s rendering decisions. MDN’s rendering overview and critical rendering path guidance explain these stages.
What is the difference between reflow and repaint?
Reflow is about geometry: recalculating where elements are and how much space they occupy. Repaint is about appearance: drawing visual details into pixels. A change can trigger both, but they are not interchangeable terms. If an element’s size or position changes, the browser commonly needs layout and then paint for affected content. If its geometry stays the same but its color changes, the browser generally needs style recalculation and paint, not layout. MDN’s browser-rendering documentation describes this distinction.
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Which updates typically need layout, paint, or composition?
The table shows typical rendering paths, not guarantees. Actual work depends on the browser, the page, and whether the browser can handle an update on a suitable independent layer.
| Example update | Typical effect | Why |
|---|---|---|
Change width |
Style recalculation, layout, and paint | The element’s geometry changes. |
Change left |
Style recalculation, layout, and paint | The element’s position changes through layout. |
Change color |
Style recalculation and paint | The appearance changes without ordinarily changing geometry. |
Change transform or opacity |
May be handled during composition on a suitable independent layer | The browser may be able to update the layer without layout or repainting it. |
These are common patterns, not a universal cost ranking for every implementation. For broader property guidance, see MDN’s CSS and JavaScript animation performance guidance.
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Why use transform instead of left for some animations?
Animating left changes layout position, so it commonly brings layout and paint work along with it. Animating transform can instead move an element visually without changing the layout box; when the browser can handle that update during composition on a suitable layer, it may avoid layout and repainting that layer. opacity can have a similar advantage.
That makes transform and opacity useful options to test for motion and fading effects. It does not mean every transform is GPU-accelerated, that every element gets its own layer, or that every CSS animation is fast. Layer creation and management have memory costs, and the browser may choose different rendering paths for different pages and devices. Measure the actual animation rather than assuming the property name settles its performance. MDN’s animation guidance covers these trade-offs.
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How much time does a frame have?
MDN describes 60 frames per second as a commonly accepted smooth-animation rate. At that rate, a frame takes about 16.7 milliseconds; that time has to accommodate scripts, style recalculation, layout when needed, and repainting. It is a reference point for 60 fps, not a universal budget for every display or device. The less work a page needs to do in each frame, the more room it has to keep updates smooth. See MDN’s animation performance and frame-rate guidance.
How can you find the expensive work on your page?
Use the browser’s performance tools to inspect what a specific interaction or animation actually triggers. Look for layout, paint, and long-running script work during the period when the page stutters or shifts; test on the target page and device, since property behavior alone cannot establish frame performance. MDN’s critical rendering path guidance recommends batching updates and avoiding animation of box-model properties where possible.
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- Batch related DOM or style updates instead of repeatedly changing geometry between reads and writes.
- Reduce unnecessary changes to dimensions and layout position.
- Consider containment or
content-visibilitywhere appropriate to limit work; these techniques do not replace profiling. See MDN’s CSS containment documentation. - Use
will-changeselectively, not as a blanket performance setting. Layer promotion can consume memory, and it is not guaranteed to help. See MDN’swill-changereference.
How can you reduce layout shifts from late content?
When an image, embed, or other delayed content has known dimensions, reserve the appropriate space before it loads. This reduces the chance that its arrival will push surrounding content into new positions. It addresses one source of unexpected movement, not every cause of layout shift. For background on layout shifts and related rendering behavior, see MDN’s browser-rendering overview.
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