The Tool Desk
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What does resetOnNavigation do?
resetOnNavigation is the only CSS coverage option documented in the Puppeteer v25.12.0 API reference. Its default, true, resets CSS coverage on each navigation. Set it to false when you want to disable that automatic reset while exercising a multi-page flow. The reference describes the reset behavior, but does not promise that this setting guarantees coverage will persist across navigations. Puppeteer CoverageOptions API reference.
| Setting | Effect | When it may fit |
|---|---|---|
resetOnNavigation: true (default) |
Resets coverage on each navigation. | When you want results scoped to individual navigations. |
resetOnNavigation: false |
Disables the automatic reset on navigation. | When collecting while exercising a flow across pages; verify the resulting reports for your workflow rather than assuming persistence is guaranteed. |
Do not confuse this CSS option with includeRawScriptCoverage, reportAnonymousScripts, or useBlockCoverage. Those options belong to JavaScript coverage, not CSS coverage.
How to start and stop CSS coverage
Start the collector before exercising the page states you want represented, then stop it to retrieve stylesheet coverage reports:
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await page.coverage.startCSSCoverage({ resetOnNavigation: false });
// Navigate and exercise the page states you want represented.
const cssCoverage = await page.coverage.stopCSSCoverage();
console.log(cssCoverage);Omitting the options object uses the default reset behavior. The stopCSSCoverage() method resolves to an array of coverage reports for stylesheets. See the Puppeteer Coverage API reference.
Derive a used-byte estimate
The official example totals observed used bytes by summing range.end - range.start - 1 for each range in each report. It compares that total with stylesheet text length to calculate a percentage. This is an observed-use metric from the run, not proof that CSS is universally unused.
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const usedBytes = cssCoverage.reduce((total, report) =>
total + report.ranges.reduce((sum, range) =>
sum + range.end - range.start - 1, 0), 0);
const totalBytes = cssCoverage.reduce((sum, report) =>
sum + report.text.length, 0);
const usedPercent = totalBytes === 0 ? 0 : (usedBytes / totalBytes) * 100;
console.log({ usedBytes, totalBytes, usedPercent });Use this as a measurement of what your test exercised. A different route, responsive breakpoint, interaction, or application state may use additional CSS.
What CSS coverage can miss
Puppeteer’s official coverage documentation states: “CSS Coverage doesn’t include dynamically injected style tags without sourceURLs.” Such injected styles may therefore be absent from the report. If your application adds styles at runtime, test the relevant pages and states and account for this limitation before removing CSS based on coverage alone.
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Troubleshooting coverage results
- Coverage appears to reset after a navigation: Check whether you are using the default
resetOnNavigation: true. Set it tofalseto disable the automatic reset, while validating the collected output for your flow. - Expected injected styles are missing: Dynamically injected style tags without sourceURLs are outside the documented CSS coverage results. Include the relevant runtime states in testing and do not treat the report as a complete inventory.
- A low used-CSS percentage seems conclusive: It only reflects styles observed in the states exercised during that run. Exercise additional routes and interactions before using it to make cleanup decisions.
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