To stop CSS coverage in Puppeteer and retrieve the measurements, call await page.coverage.stopCSSCoverage() after the navigation and interactions you want to measure. It resolves to an array of stylesheet coverage entries; it does not remove CSS or decide what is safe to delete.
Stop coverage after the page activity you want to measure
Start CSS coverage before navigating or interacting with the page. Stop it when the relevant activity is complete:
const page = await browser.newPage();
await page.coverage.startCSSCoverage();
await page.goto('https://example.com');
// Exercise the routes, controls, menus, and states you want to include.
const entries = await page.coverage.stopCSSCoverage();
for (const entry of entries) {
console.log(entry.url, entry.ranges);
}The public page-level workflow is documented in Puppeteer’s Coverage class example. The stop method’s documented result is CoverageEntry[], with reports for stylesheets; see Coverage.stopCSSCoverage().
What the returned entries tell you
Each entry provides stylesheet information, including its URL, text, and ranges identified as used during the captured run. Those ranges are evidence about the page activity you exercised, not a ready-made CSS-removal plan. A page load that never opens a menu, visits another route, or triggers a responsive state cannot show whether styles used by those states are needed.
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Estimate the used-byte percentage
Puppeteer’s class documentation demonstrates calculating a byte-based percentage from the returned entries. For each entry, count entry.text.length as total text length and sum each used range as range.end - range.start - 1; divide the used total by the total text length and multiply by 100.
const totalBytes = entries.reduce((sum, entry) => sum + entry.text.length, 0);
const usedBytes = entries.reduce(
(sum, entry) => sum + entry.ranges.reduce(
(entrySum, range) => entrySum + range.end - range.start - 1,
0
),
0
);
const usedPercent = totalBytes === 0 ? 0 : (usedBytes / totalBytes) * 100;
console.log({ totalBytes, usedBytes, usedPercent });This is a calculation for the captured run, not a benchmark or proof that the remaining CSS can be deleted. Validate candidate removals across the site’s routes and UI states.
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Navigation resets and collection boundaries
startCSSCoverage() has a documented resetOnNavigation option whose default is true. If your measurement depends on how coverage behaves across navigation, set the option deliberately and check the documentation for your installed Puppeteer version: Coverage.startCSSCoverage().
For the most representative measurement, decide whether you want one page state or a sequence of interactions and routes, then keep the collection boundaries aligned with that goal. A single navigation is not a site-wide inventory.
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Why injected styles may be missing
Puppeteer’s stop-method documentation notes: “CSS Coverage doesn’t include dynamically injected style tags without sourceURLs.” If a page creates style tags at runtime and they lack source URLs, their CSS may not appear in the report. This limitation can explain why an injected style is absent even when it visibly affects the page.
Troubleshooting coverage results
- The result is empty or unexpectedly small: confirm that coverage was started before the activity, then inspect whether the relevant stylesheet loaded during that run.
- Styles used by a menu or dialog appear unused: trigger that UI before stopping coverage; only exercised states contribute to the observation.
- Injected CSS is missing: check whether it is in a dynamically injected style tag without a sourceURL, which the API documentation identifies as outside CSS coverage.
- Results appear reset after navigation: review the
resetOnNavigationsetting passed tostartCSSCoverage()and match it to the behavior you need. - You are about to delete reported-unused rules: treat the report as a lead, then validate changes against routes, interaction states, and responsive layouts that matter to the site.
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