Eliminate render-blocking JavaScript and CSS - advice needed

Question

Google PageSpeed Insights is flagging this as something I should fix - I've read the guidance on Optimising CSS Delivery at https://developers.google.com/speed/docs/insights/OptimizeCSSDelivery but I'm confused at what the best practice is, and also on which resources are render blocking and which aren't?

Is Google suggesting removing stylesheet links from the page head and replacing with inline styles to make something render, then using Javascript to trigger an external stylesheet to load when window.onload fires? Won't this just delay process of arriving at the 'correctly rendered' page - isn't it better for the browser to start downloading the CSS as soon as possible?

Answer 1

Yes, that's pretty much what the page you reference is recommending. Put the minimal amount of CSS (as long as it's a small amount) directly in the HTML markup within a <style> tag. Then include the complete set of styles at the end of the document. (In the example, it's not actually loaded via JavaScript per se; rather, the link to the external style sheet is placed in a <noscript> tag. That's a bit of a hack, but it gets the job done. You could also request the stylesheet via AJAX and inject it using JavaScript directly.)

This approach only works if you can isolate the minimal CSS needed for your page and that amount of CSS is reasonably small. If, for example, you're building a single page web app, then many of your CSS rules might be for parts of the app other than the initial view. In that case, those extra rules can be put in the external style sheet. Or maybe you have a set of rules strictly for pop-up dialog boxes. Those rules can be postponed as well.

If you can't really separate your rules into those that are needed initially and those that aren't, and if your minimal rule set is large, you can't take advantage of this approach.