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Update LayoutShiftAttribution documentation for impact-area ordering #42145

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sideshowbarker merged 3 commits into from
Dec 2, 2025
Merged

Update LayoutShiftAttribution documentation for impact-area ordering #42145

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9fd5460
update layout shift documentation
anediaz Nov 27, 2025
32ebc3c
Fix linting errors in LayoutShiftAttribution docs
anediaz Dec 2, 2025
0d4b3dd
Update files/en-us/web/api/layoutshiftattribution/index.md
sideshowbarker Dec 2, 2025
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27 changes: 13 additions & 14 deletions files/en-us/web/api/layoutshiftattribution/index.md
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Expand Up @@ -29,23 +29,22 @@ Instances of `LayoutShiftAttribution` are returned in an array by calling {{domx

## Examples

The following example finds the element with the highest layout shift score, and returns the element in that entry with the largest size prior to the shift (`previousRect`). For more detail on this see [Debug Web Vitals in the field](https://web.dev/articles/debug-performance-in-the-field).
The following example observes layout shifts and identifies the most impactful element. The `sources` array is sorted by impact area, in descending order — so the first element (`sources[0]`) represents the element that contributed most to the layout shift. For more detail on that, see [Debug Web Vitals in the field](https://web.dev/articles/debug-performance-in-the-field).

```js
function getCLSDebugTarget(entries) {
const largestEntry = entries.reduce((a, b) =>
a && a.value > b.value ? a : b,
);
if (largestEntry?.sources?.length) {
const largestSource = largestEntry.sources.reduce((a, b) => {
const area = (el) => el.previousRect.width * el.previousRect.height;
return a.node && area(a) > area(b) ? a : b;
});
if (largestSource) {
return largestSource.node;
}
const observer = new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
if (!entry.sources || entry.sources.length === 0) continue;

const mostImpactfulSource = entry.sources[0];
console.log("Layout shift score:", entry.value);
console.log("Most impactful element:", largestShiftSource.node);
console.log("Previous position:", largestShiftSource.previousRect);
console.log("Current position:", largestShiftSource.currentRect);
}
}
});

observer.observe({ type: "layout-shift", buffered: true });
```

## Specifications
Expand Down