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1.1.3 • Public • Published

CCSSS - Critical-path CSS Service


Yet another critical-path CSS tool. Don't know what it's for? Please refer to this article.

In brief:

  • it's an API
  • that will extract the critical-path CSS of your Web pages
  • and return it to your website
  • so that generation and inclusion of that CSS is automated.

Why another tool

Some existing solutions such as Addy Osmani's critical will require you to have a static version of your pages to run the tool during your build.
For dynamic websites, this won't do, obviously.

At Malt, we ran a custom solution that reused our Selenium test harness to dump the HTML code of our pages for critical to work on it, but it was still quite unpractical, especially since the page look depends on a lot of data that have to be reproduced for those test runs.

Other solutions such as criticalcss.com will let you enter your Web page addresses and return you the critical-path CSS, but it can't be automated, which make it a no-go.

Finally, Google PageSpeed module for Apache and Nginx is able to generate and inline critical-path CSS when proxying requests, but it has a number of limitations (doesn't work for IE, handle all requests whether the user agent already had the opportunity to cache regular CSS files or not, etc.)

So, this tool relies on penthouse, as the other tools quoted, but it provides a way to get the critical-path CSS that corresponds to your Web page, while allowing for automating the generation and inclusion of that CSS into the website. Of course, it can't be part of your build as it will hit the real website, but it can be part of your post-deployment tasks, though.


Install ccsss (globally in this example):

$ sudo npm install --global --production

Start ccsss:

$ ccsss --port 1234

The port may be omitted, ccsss will run on port 8888 by default.

Post a critical-path CSS generation request:

$ curl -v -H "Content-Type: application/json" -X POST -d '{
    // mandatory
    "url": "http://mywebsite.org/some/page",
    "dimensions": [{
        "width": 1280,
        "height": 800
    }, {
        "width": 320,
        "height": 568
    // optional
    "forceInclude": ["#normallyHiddenId"],
    "forceIncludeRe": [".important-.*"],
    "ignore": ["font-face", ".some-class", "form"],
    "ignoreRe": ["some.*regular.*expression"],
    "notificationUrl": "http://mywebsite.org/notification/critical-css-ready",
    "phantomJsOptions": { // see penthouse documentation and `phantomjs --help` for the list of all available options
        "proxy": "http://proxy.company.com:8080",
        "ssl-protocol": "SSLv3"
    "userAgent": "Some customer User-Agent that ccsss will use when browsing `url`"  // defaults to "ccsss"
}' http://localhost:8888/generation/request

ccsss will return you something like the following:

< HTTP/1.1 202 Accepted
< Location: http://localhost:8888/generation/result/47edab4c-caa7-4dc9-987a-f04716ed009f


The Location header tells you where the result will be available once the generation is over. The JSON response also gives you a generation ID so you can later on associate notifications with the requests you made.

Assuming you do have a endpoint at http://mywebsite.org/notification/critical-css-ready, you will receive a notification like the following one once the result is available:

    "generationId": "1103f981-8052-4d3a-b098-d8f6c3eede22",
    "status": "success",
    "resultLocation": "http://localhost:8888/generation/result/47edab4c-caa7-4dc9-987a-f04716ed009f"

You can then get you result. Note that having a notificationUrl is optional and that you can also blindly poll the given result URL from the beginning.

$ curl -v http://localhost:8888/generation/result/47edab4c-caa7-4dc9-987a-f04716ed009f
< HTTP/1.1 200 OK
< Content-Type: text/css

#myElement,.my-class{position:relative}code,h1,h2,h3{word-wrap:break-word} /* ... */

Finally, please note that once a result has been consumed, it is forgotten by the server:

$ curl -v http://localhost:8888/generation/result/47edab4c-caa7-4dc9-987a-f04716ed009f
< HTTP/1.1 404 Not Found
Not found

Example: integration within a webapp

Yet to be written


npm i ccsss

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