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Generate and inline critical-path CSS example usingCritical.

Live demo ofbefore andafter critical-path CSS generation and inlining.

PageSpeed Insights results of before and after

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After:

WebPageTest results

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After:

UseCritical for generating and inlining your critical-path CSS andloadCSS toasync load in yoursite-wide styles.

CSS is required to construct the render tree for your pages and JavaScript will often block on CSS during initial construction of the page. You should ensure that any non-essential CSS is marked as non-critical (e.g. print and other media queries), and that the amount of critical CSS and the time to deliver it is as small as possible.

For best performance, you may want to consider inlining the critical CSS directly into the HTML document. This eliminates additional roundtrips in the critical path and if done correctly can be used to deliver a “one roundtrip” critical path length where only the HTML is a blocking resource.

$cd critical-path-css-demo$ npm install&& bower install

Note: There are two build commands available. This allows you to compare the differencebetween the output of a normal build and the output with critical-path CSS.

The default (minify, concat) build for the project can be run with:

$ gulp

The complete (critical-path) build can be run with:

$ gulp critical

This performs the normal build, then generates and inlines critical-path CSS for the page. It automatically async loads in the site-wide styles usingloadCSS as part of the workflow offered by the module.

Follow the next few lines to install and scaffold a project usingYeoman and Gulp:

$ mkdir myapp&&cd myapp$ npm install -g yo generator-gulp-webapp$ yo gulp-webapp# Select Bootstrap and say no to Modernizr & Sass

You should now have a valid set of source files, including aGulpfile.js.

The first thing we're going to do is install the Critical module which can generate and inline your critical-path CSS for you.

This can be installed as follows:

$cd myapp$ npm install critical --save-dev

Great. Next, add a reference to Critical at the top of yourGulpfile.js:

varcritical=require('critical');

We can now use it in our build process. Let's write a new task calledcritical.

Our workflow for critical-path CSS is to first run a normalbuild, which will generate the optimized CSS (dist/styles/main.css) and resources needed for our app. We pass thebuild command as the second argument below:

gulp.task('critical',['build'],function(){});

Next, we'll add in our configuration for thecritical module:

gulp.task('critical',['build'],function(cb){critical.generate({inline:true,base:'dist/',src:'index.html',dest:'dist/index-critical.html',minify:true,width:320,height:480});});

That's it. You can now rungulp critical to generate a complete build wheredist/index-critical.html will contain your final output files. Above I've passed in awidth andheight which represent the viewports I'm targeting with my above-the-fold CSS.minify ensures that the inlined CSS gets minified.

Note that this sample project is just that - a sample. It does not demonstrate how well these tools andtechniques work on a complex site nor a site making heavy use of dynamic styles. Your mileage may varyand I encourage testing the tools available before making a decision about whether Critical makes sensefor you.