Lighthouse flags two types of render-blocking URLs: scripts and stylesheets.
<script> tag that:
<head>of the document.
<link rel="stylesheet"> tag that:
disabledattribute. When this attribute is present, the browser does not download the stylesheet.
mediaattribute that matches the user's device.
The first step to reducing the impact of render-blocking resources, is to identify what's critical and what's not. Use the Coverage tab in Chrome DevTools to identify non-critical CSS and JS. When you load or run a page, the tab tells you how much code was used, versus how much was loaded:
Once you've identified critical code, move that code from the render-blocking URL to an inline
script tag in your HTML page. When the page loads, it will have what it needs to handle the page's core functionality.
If there's code in a render-blocking URL that's not critical, you can keep it in the URL, and then mark the URL with
Code that isn't being used at all should be removed (see Remove unused code).
Similar to inlining code in a
<script> tag, inline critical styles required for the first paint inside a
<style> block at the
head of the HTML page. Then load the rest of the styles asynchronously using the
preload link (see Defer unused CSS).
Consider automating the process of extracting and inlining "Above the Fold" CSS using the Critical tool.
Another approach to eliminating render-blocking styles is to split up those styles into different files, organized by media query. Then add a media attribute to each stylesheet link. When loading a page, the browser only blocks the first paint to retrieve the stylesheets that match the user's device (see Render-Blocking CSS).
Finally, you'll want to minify your CSS to remove any extra whitespace or characters (see Minify CSS). This ensures that you're sending the smallest possible bundle to your users.
Use tools such as AMP Optimizer to server-side render AMP layouts.
There are a number of Joomla plugins that can help you inline critical assets or defer less important resources.
There are a number of WordPress plugins that can help you inline critical assets or defer less important resources.