Eliminate render-blocking CSS in above-the-fold content

If you've run Google Pagespeed Insights on your web app, you might have seen this message:

Eliminate render-blocking JavaScript and CSS in above-the-fold content

Your page has blocking CSS resources. This causes a delay in rendering your page. None of the above-the-fold content on your page could be rendered without waiting for the following resources to load. Try to defer or asynchronously load blocking resources, or inline the critical portions of those resources directly in the HTML.

This server generates the critical path CSS for you. It is designed to sit alongside your production app, and prepare the critical CSS asynchronously.

Builds can be started after each deploy, and a few seconds later, you'll be able to defer or asynchronously load blocking resources, and inline the critical portions for your page.

Install

npm install critical-path-server

Usage

Start the server

npm start
> Listening on port: 8080

Send a request with details about the page you want cached

The first request returns immediately, and starts generating the CSS in the background:

curl -H "Content-Type: application/json" -X POST -d '{ "page": {"key":"unique-key","url":"http://www.example.com/","css":"https://www.example.com/style.css"}}' localhost:5000/api/v1/css
> Accepted

Eventually...

curl -H "Content-Type: application/json" -X POST -d '{ "page": {"key":"unique-key","url":"http://www.example.com/","css":"https://www.example.com/style.css"}}' localhost:5000/api/v1/css
> .your-critical-css {}

Ruby on Rails

Use the Ruby on Rails client to manage rewriting and lazy-loading your other CSS assets, once the critical path has been compiled and is available.

RailsCriticalCssServer

Docker

This repo is setup with docker. You can get it up and running with the following commands.

docker build -t critical-css-server .
docker-compose up critical-css