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Start CSS coverage on a Puppeteer Page before navigating to the page or exercising the states you want to measure: await page.coverage.startCSSCoverage(). After those actions, call await page.coverage.stopCSSCoverage() to receive stylesheet reports and inspect their ranges.
Start coverage before the page actions you want to measure
CSS coverage records which portions of stylesheets Puppeteer observed as used during the collection window. The sequence matters: start collection first, then navigate and exercise the page, and stop only when you have finished those actions.
import puppeteer from 'puppeteer';
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
// Begin collection before navigating or interacting.
await page.coverage.startCSSCoverage();
await page.goto('https://example.com');
// Exercise the page states whose CSS you want to measure.
// For example, click menus, open dialogs, or scroll to lazy content.
const cssCoverage = await page.coverage.stopCSSCoverage();
console.log(cssCoverage);
} finally {
await browser.close();
}startCSSCoverage() returns a promise that resolves when collection has started. Await it before proceeding. stopCSSCoverage() resolves to an array of coverage reports, one entry for each stylesheet represented in the results.
What the coverage result contains
Each CSS report entry includes stylesheet text and ranges identifying portions observed during the collection. Puppeteer’s documentation example calculates a used-byte percentage by adding the stylesheet text lengths for total bytes and summing range.end - range.start - 1 for used bytes, then dividing used bytes by total bytes.
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const totalBytes = cssCoverage.reduce(
(sum, entry) => sum + entry.text.length,
0,
);
const usedBytes = cssCoverage.reduce(
(sum, entry) =>
sum + entry.ranges.reduce(
(rangeSum, range) => rangeSum + range.end - range.start - 1,
0,
),
0,
);
const usedPercentage = totalBytes === 0
? 0
: (usedBytes / totalBytes) * 100;
console.log({ totalBytes, usedBytes, usedPercentage });This is the byte-based calculation shown in Puppeteer’s example, not a universal measure of stylesheet quality or page performance. The percentage depends on the page states you exercised during collection, so it should be interpreted in the context of that run.
Choose navigation reset behavior deliberately
startCSSCoverage() accepts an optional CSSCoverageOptions object. Its resetOnNavigation option defaults to true.
Rank #2
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await page.coverage.startCSSCoverage({
resetOnNavigation: false,
});Keep the default unless you have a reason to change it. The documented default is useful to know when a run includes navigations; setting the option to false should not be treated as a guarantee that every stylesheet report or range will be retained across every navigation.
Important limitation: dynamically injected style tags
Puppeteer’s Coverage class documentation warns: “CSS Coverage doesn’t include dynamically injected style tags without sourceURLs.” Therefore, coverage should not be presented as a complete record of every CSS rule that may exist dynamically at runtime. If your application injects styles, check whether those styles are represented in the reports before using the results to make cleanup decisions.
The Tool Desk
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Exporting results for Istanbul
If you need an Istanbul-compatible downstream output, Puppeteer points to puppeteer-to-istanbul. It is an optional conversion path; it is not required to start CSS coverage or retrieve the reports from Puppeteer.
Troubleshooting CSS coverage
- The result is empty or missing styles you expected: confirm that
startCSSCoverage()was awaited before navigation and before the relevant interactions, and thatstopCSSCoverage()was not called before those actions completed. - A menu, dialog, or other state’s styles are absent: collection only reflects the actions and page states exercised before stopping. Trigger the state while coverage is running.
- Injected styles do not appear: dynamically injected style tags without
sourceURLannotations are excluded according to Puppeteer’s documented caveat. - Coverage appears to change around navigation:
resetOnNavigationdefaults totrue. If you change it, do not assume that doing so guarantees retained reports across navigations. - The used-byte percentage seems misleading: treat the documentation’s percentage as a byte-based calculation for the captured run, not as a quality or performance score.
The current Puppeteer API reference labels startCSSCoverage() as version 25.10.0. That is a documentation version label; compatibility requirements across Puppeteer, Node.js, and browser versions are not established here, so check the API documentation for the version installed in your project.
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Or skip the browser setup
ScreenshotNeo is a website screenshot API and MCP server, not a CSS coverage collector; use Puppeteer coverage when you need stylesheet usage ranges. If you also need clean page screenshots, a single GET can capture a URL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpSee the ScreenshotNeo API documentation for request options. ScreenshotNeo removes cookie banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, and failed loads are never billed. Its MCP server lets AI agents take screenshots. The Free plan includes 1,000 screenshots a month with no card, and paid plans start at $5 for 3,000 shots.
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Frequently Asked Questions
Does CSS coverage start automatically when I create a Puppeteer page?
No. Call and await page.coverage.startCSSCoverage() explicitly.
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Can I use the returned data with Istanbul?
Yes. Puppeteer identifies puppeteer-to-istanbul as an optional path for Istanbul-compatible output.
Quick Recap
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