Back to Community
Home / Community / Plugin → Autoptimize

Resolving Render-Blocking CSS Issues with Autoptimize

14 threads Sep 9, 2025 Plugin → Autoptimize

Content

If you've configured Autoptimize to optimize your CSS and JavaScript but are still seeing warnings about render-blocking resources in tools like Google Lighthouse, you're not alone. This is a common point of confusion. This guide will explain why this happens and walk you through the most effective solutions.

Why Does Autoptimize Create Render-Blocking CSS?

When you enable the "Optimize CSS Code" and "Aggregate CSS" options, Autoptimize combines your site's multiple CSS files into one larger file. This reduces the number of HTTP requests, which is good for performance. However, by default, browsers treat all CSS as a render-blocking resource. This means the browser must download, parse, and process this combined CSS file before it can render the page to the user, which can delay key metrics like Largest Contentful Paint (LCP). The goal is to manage this process, not eliminate the CSS file itself.

The Primary Solution: The "Eliminate Render-Blocking CSS" Option

The most effective way to tackle this issue is within Autoptimize itself. The plugin includes a powerful feature specifically designed for this problem. Here’s how to use it:

  1. Navigate to Autoptimize → CSS Settings in your WordPress admin dashboard.
  2. Locate and check the option labeled "Inline and Defer CSS" (often described as "eliminate render-blocking CSS" in support threads).
  3. This option changes how the optimized CSS is delivered. It will inline a small snippet of "critical" CSS directly in your page's HTML and load the main, large CSS file asynchronously.

However, simply checking this box is only half the solution. For it to work correctly, you must also provide the critical CSS that will be inlined.

How to Generate and Provide Critical CSS

Critical CSS is the minimal set of CSS styles needed to render the visible portion of the page (the "above-the-fold" content). You have a few options to handle this:

  • Manual Method: Use a free online tool like jonassebastianohlsson.com/criticalpathcssgenerator or others mentioned in the Autoptimize FAQ. You will need to generate critical CSS for each different type of page template on your site (e.g., homepage, blog post, product page). You can then add these rules manually on the "Critical CSS" tab in Autoptimize's settings.
  • Automatic Service: For a more hands-off approach, you can integrate a third-party service like criticalcss.com directly from the "Critical CSS" tab. This can automatically generate and manage critical CSS rules for all your page types.

Other Common Scenarios and Fixes

  • New Pages Aren't Optimized: If you create a new page and find CSS is still render-blocking, the most likely cause is that there is no matching critical CSS rule for that page template. You will need to create a new rule for it on the Critical CSS tab. Additionally, ensure you clear any page cache you might be using after creating new rules.
  • Large CSS File Competing with LCP Image: Even if not render-blocking, a very large CSS file can compete for bandwidth with your LCP image, delaying its load. Using the "Inline and Defer CSS" method also helps here, as it drastically reduces the priority of the main CSS file.
  • Plugin Conflicts: In some cases, other optimization plugins (like WP Rocket) can interfere. One thread indicated that WP Rocket was adding an attribute to an Autoptimized file. If you use multiple optimization plugins, try configuring them to not optimize the same assets (e.g., let Autoptimize handle JS and another plugin handle CSS, or vice-versa) to avoid conflict.

What About JavaScript?

It's important to note that the "Inline and Defer CSS" option only affects CSS. For JavaScript, ensure the "Optimize JavaScript Code" option is enabled. You can also experiment with the "Defer" and "Aggregate" settings under JavaScript options, but be aware that deferring JavaScript can sometimes break scripts that need to execute early.

By understanding how Autoptimize works and properly configuring the "Inline and Defer CSS" feature with the necessary critical CSS, you can effectively eliminate render-blocking CSS and improve your site's loading performance.

Related Support Threads Support