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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →To measure CSS exercised by a Puppeteer run, start collection with page.coverage.startCSSCoverage() before navigating or interacting with the page, then call page.coverage.stopCSSCoverage(). The returned stylesheet entries include the source text and covered character ranges, which you can sum to estimate the share observed during that run. It is evidence about the routes and states you tested—not proof that every unobserved rule is safe to delete.
Collect CSS coverage with Puppeteer
Start coverage before the browser activity you want to include. The example below navigates to a page, stops collection, and prints a byte-range-style usage estimate for the captured CSS entries.
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const { chromium } = require('puppeteer');
(async () => {
const browser = await chromium.launch();
try {
const page = await browser.newPage();
await page.coverage.startCSSCoverage();
await page.goto('https://example.com');
// Perform relevant interactions here before stopping coverage.
const cssCoverage = await page.coverage.stopCSSCoverage();
let totalChars = 0;
let usedChars = 0;
for (const entry of cssCoverage) {
totalChars += entry.text.length;
for (const range of entry.ranges) {
usedChars += range.end - range.start - 1;
}
}
const percentUsed = totalChars === 0 ? 0 : (usedChars / totalChars) * 100;
console.log(`Estimated CSS covered in this run: ${percentUsed.toFixed(2)}%`);
console.log(cssCoverage);
} finally {
await browser.close();
}
})();This uses Puppeteer’s documented range-length expression. The zero-length guard prevents division by zero if no stylesheet text was returned. The percentage is a character-range estimate, not a selector count, rule count, or assurance that the remaining CSS can be removed.
The Tool Desk
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Coverage only records browser activity observed while collection is active. A single landing-page load may miss CSS used on other routes, in responsive layouts, or after user actions. Before using the result to guide cleanup, exercise the application states you need to support.
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- Visit each relevant route while coverage is active.
- Use representative viewport sizes and device conditions.
- Open menus, dialogs, accordions, and other interactive states.
- Wait for delayed content or trigger states that load after user input.
- Repeat with different scenarios when a single run cannot cover the application’s important states.
Choose how navigation affects collection
page.coverage.startCSSCoverage() accepts an optional CSSCoverageOptions object. Its documented navigation option is resetOnNavigation; the Puppeteer start-method reference lists the default as true.
await page.coverage.startCSSCoverage({ resetOnNavigation: false });With the documented default, coverage resets on navigation. Set the option explicitly when your measurement spans navigations and you need coverage retained across them. Check the API reference for the Puppeteer version installed in your project if exact behavior is consequential, because versioned signatures and defaults can change.
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Read the coverage entries
stopCSSCoverage() resolves to an array of stylesheet reports. Each CoverageEntry documents these fields:
url: the stylesheet or script URL; for CSS coverage, use the stylesheet entries returned by the CSS method.text: the stylesheet or script content.ranges: an array of covered ranges, each withstartandendoffsets.
For a CSS-only percentage, sum the lengths of the ranges in the CSS coverage result and divide by the combined length of the entry text. Keep the raw entries as well as the derived percentage: the URLs and ranges help identify which source files contributed to the measurement.
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What “unused CSS” means here
A range not present among the covered ranges was not observed in that run. It may still be needed on a route, viewport, or interaction you did not exercise. Treat the output as a test-run measurement, not an automatic safe-to-delete list.
Puppeteer documents a specific limitation: “CSS Coverage doesn’t include dynamically injected style tags without sourceURLs.” Consider that when applications create styles dynamically; the resulting report may not represent all styles applied by the page.
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Troubleshoot missing or surprising results
The returned array is empty or contains no useful CSS
- Confirm that
startCSSCoverage()ran before the page activity being measured. - Check that the page loaded stylesheets during the collection period.
- Check the returned entries and their URLs to determine what was captured.
The percentage seems too low
- Make sure the test exercised the routes, viewport conditions, and interactions relevant to your application.
- Check whether
resetOnNavigationis resetting data between navigations; set it explicitly if the workflow needs coverage retained across them. - Remember that dynamically injected style tags without sourceURLs are excluded from CSS coverage.
The result changes between runs
Compare the browser activity in each run. Different routes, viewport sizes, or interactions can expose different CSS ranges, so a percentage is meaningful only in the context of the scenario that produced it.
Or skip the browser setup
If you need a rendered page screenshot rather than CSS coverage data, ScreenshotNeo provides a website screenshot API and MCP server. It does not measure CSS coverage; it captures a page as an image or PDF.
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Documentation and version context
The versioned Puppeteer API references consulted identify version 25.12.0 for the Coverage and CSSCoverageOptions pages, and version 25.10.0 for CoverageEntry. Check the documentation for the version you use when relying on a particular signature or default:
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