@plone/critical-css-cli

A command-line utility to generate critical CSS from a set of URLs. The functionality is useful for a CMS-powered website, where pages are user-defined and and their CSS comes from multiple addons. Because the website layout is generated based on the CMS content, it would be impossible to understand what is Critical CSS at build time, only once the website is moved to production and the "dust settles" on the main pages structure.

Typically, for the best possible performance, you'd want to generate the critical CSS as short as possible and that means one CSS per separate page. But this complicates things: async runners, file refresh, file invalidations, etc, would have to be taken into account. This raises the barrier of entry for the project.

This package tries to keep things really simple by accepting some limitations. A single CSS file is generated based on multiple provided URLs. The CSS is optimized automatically using postcss and cssnano, so duplicated rules are eliminated.

Install globally

npm install -g @plone/critical-css-cli

Then you can use it like:

critical-cli -h

Install from this repo

Either install it as a native package with: yarn install or npm install.

Then run the generator like:

bin/critical-cli -o ./out/critical.css https://plone.org

This will generate a file called critical.css in the out folder.

Run bin/critical-generator -h to see full options.

Use via docker

Alternatively, you can use docker to build and use this package. Pass website url for which critical css has to be generated as env var to docker-compose

URL=<website-url> docker-compose up

Config

Pass the DIMENSIONS env variable if you want to deliver critical CSS for multiple screen resolutions. Defaults to 1300x900 if you don't want.

URL=<website-url> DIMENSIONS="1220x300,1345x500" docker-compose up

Location to save the critical.css file. Defaults to /var/app/output/critical.css (inside the docker container)

URL=<website-url> LOCATION="output/<your-dir>/critical.css" docker-compose up

The location is relative to the current directory.