2.3.3 • Public • Published


Critical Path CSS Generator

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Penthouse is the original critical path css generator, helping you out to speed up page rendering for your websites. Supply your site's full CSS and the page you want to create the critical CSS for, and Penthouse will return the critical CSS needed to perfectly render the above the fold content of the page. Read more about critical path css here.

The process is automatic and the generated CSS is production ready as is. Behind the scenes Penthouse is using puppeteer to generate the critical css via the chromium:headless.


(If you don’t want to write code, you can use the online hosted version.)

yarn add --dev penthouse

(or npm install, if not using yarn)

Basic example

  url: '',
  cssString: 'body { color: red }'
.then(criticalCss => {
  // use the critical css
  fs.writeFileSync('outfile.css', criticalCss);

More examples

Performance when running many jobs

Penthouse is optimised for running many jobs in parallel. One shared browser instance is re-used and each job runs in its own browser tab. There's only so many jobs you can run in parallel before your machine starts running out of resources. To run many jobs effectively, see the many urls example.


Only url and cssString are required - all other options are optional. Note that the html file found via url is expected to be styled; penthouse does not inject any styles, it just uses cssString (or css) to prune into critical css.

Name Type Default Description
url string Accessible url. Use file:/// protocol for local html files.
cssString string Original css to extract critical css from
css string Path to original css file on disk (if using instead of cssString)
width integer 1300 Width for critical viewport
height integer 900 Height for critical viewport
screenshots object Configuration for screenshots (not used by default). See Screenshot example
keepLargerMediaQueries boolean false Keep media queries even for width/height values larger than critical viewport.
forceInclude array [] Array of css selectors to keep in critical css, even if not appearing in critical viewport. Strings or regex (f.e. ['.keepMeEvenIfNotSeenInDom', /^\.button/])
forceExclude array [] Array of css selectors to remove in critical css, even if appearing in critical viewport. Strings or regex (f.e. ['.doNotKeepMeEvenIfNotSeenInDom', /^\.button/])
propertiesToRemove array ['(.*)transition(.*)', 'cursor', 'pointer-events', '(-webkit-)?tap-highlight-color', '(.*)user-select'] ] Css properties to filter out from critical css
timeout integer 30000 Ms; abort critical CSS generation after this time
puppeteer object Settings for puppeteer. See Custom puppeteer browser example
pageLoadSkipTimeout integer 0 Ms; stop waiting for page load after this time (for sites when page load event is unreliable)
renderWaitTime integer 100 ms; wait time after page load before critical css extraction starts (also before "before" screenshot is taken, if used)
blockJSRequests boolean true set to false to load JS (not recommended)
maxEmbeddedBase64Length integer 1000 characters; strip out inline base64 encoded resources larger than this
maxElementsToCheckPerSelector integer undefined Can be specified to limit nr of elements to inspect per css selector, reducing execution time.
userAgent string 'Penthouse Critical Path CSS Generator' specify which user agent string when loading the page
customPageHeaders object Set extra http headers to be sent with the request for the url.
cookies array [] For formatting of each cookie, see Puppeteer setCookie docs
strict boolean false Make Penthouse throw on errors parsing the original CSS. Legacy option, not recommended.
allowedResponseCode number|regex|function Let Penthouse stop if the server response code is not matching this value. number and regex types are tested against the response.status(). A function is also allowed and gets Response as argument. The function should return a boolean.


Enable debug logging

Logging is done via the debug module under the penthouse namespace. You can view more about the logging on their documentation.

# Basic verbose logging for all components
env DEBUG="penthouse,penthouse:*" node script.js

Not working on Linux

Install missing dependencies to get the headless Chrome to run:

sudo apt-get install libnss3

You might possibly need an even longer list of deps, depending on your dist, see this answer

Problems with generated CSS

A good first step can be to test your url + css in the hosted critical path css generator, to determine whether the problem is with the input your passing (css + url), or with your local setup:

Unstyled content showing when using the critical css

If you see flashes of unstyled content showing after applying your critical css then something is wrong. Below are the most commont causes and some general related advice:

Your page contains dynamic or JS injected/activated content.

Generally you have to ensure that all elements you want styled in the critical css appears (visible) in the HTML of your page (with Javascript disabled). The first render of your page, the one critical css helps make much faster, happens before JS has loaded, which is why Penthouse runs with JavaScript disabled. So if your html is essentially empty, or your real content is hidden before a loading spinner or similar you have to adress this before you can get the performance benefits of using critical css.

If your html is fine, but varies based on things such as the logged in user, third party advertising etc, then you can use the forceInclude parameter to force specific extra styles to remain in the critical css, even if Penthouse doesn’t see them on the page during critical css generation.

Early DOM content moved out of critical viewport via CSS

This problem can happen if you have an element appearing early in the DOM, but with styles applied to move outside of the critical viewport (using absolute position or transforms). Penthouse does not look at the absolute position and transform values and will just see the element as not being part of the critical viewport, and hence Penthouse will not consider it’s styles critical (so the unstyled element will show when the critical css is used). Solution: Consider whether it really should appear so early in the DOM, or use the forceInclude property to make sure the styles to "hide"/move it are left in the critical css.

Special glyphs not showing/showing incorrectly

Problems with special characters like → after converting? Make sure you use the correct hexadecimal format in your CSS. You can always get this format from your browser console, by entering '→'.charCodeAt(0).toString(16) (answer for this arrow glyph is 2192). When using hexadecimal format in CSS it needs to be prepended with a backslash, like so: \2192 (f.e. content: '\2192';)

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