Imagine you're going to a theater to see a play. Even though all the actors are ready backstage, the curtain won't rise until the lighting technician and sound engineer finish their setup. In this scenario, the lighting and sound setup are "render blocking"—they prevent the show from starting, even though everything else is ready.
On your website, render blocking resources work the same way. They're files (typically CSS and JavaScript) that prevent your page from displaying any visual content until they've been fully downloaded, parsed, and executed. While these resources are processing, your visitors see nothing but a blank white screen, regardless of how fast the rest of your page might load.
Render blocking resources directly impact how quickly visitors can see and interact with your website:
The frustrating part is that many websites delay showing any content not because the content itself isn't ready, but because they're forcing the browser to load resources that aren't immediately necessary for the initial view.
To understand render blocking resources, it helps to know how browsers actually display web pages:
The key insight is that the browser is technically capable of showing content before all resources are loaded, but certain resources force it to wait, creating that frustrating blank screen period.
Browsers block rendering for good reason—CSS could completely change how the page looks, and JavaScript might modify the DOM structure. Without waiting, the browser risks showing content that immediately changes or jumps around, creating a poor user experience. The challenge is finding the right balance between caution and speed.
Not all resources block rendering. Here's what you need to know about which ones do:
Resources that typically don't block rendering include:
To find render blocking resources on your website, you'll need to analyze how your page loads:
When identifying render blocking resources, pay special attention to:
External CSS files block rendering until they're downloaded and processed, even if most styles aren't needed for above-the-fold content.
Simple fix: Extract the CSS needed for above-the-fold content and include it directly in your HTML's <head> section. Then load the full CSS files in a non-blocking way.
Not all CSS is needed immediately—styles for footers, lower page sections, or alternate device sizes can load later.
Simple fix: Load non-critical CSS using a pattern like this: <link rel="preload" href="styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
CSS files with media queries only block rendering if they match the current device type.
Simple fix: Add appropriate media queries to your CSS links, like media="print" for print stylesheets or media="(min-width: 1200px)" for desktop-only styles. The browser will still download these but won't block rendering when the media type doesn't match.
JavaScript in the document head blocks rendering by default.
Simple fix: Add the async attribute to scripts that don't need to run in a specific order, or defer for scripts that should run in order but after HTML parsing is complete.
Scripts placed just before the closing </body> tag naturally load after the HTML has been parsed.
Simple fix: For scripts that aren't needed for initial rendering, move them from the <head> to just before the </body> tag. This lets the browser render content before loading and executing these scripts.
While not technically render blocking, web fonts can delay text visibility.
Simple fix: Use font-display: swap in your CSS @font-face rules to show text with a system font while custom fonts load. Preload critical fonts and consider using system font stacks for better performance.
Multiple small CSS or JavaScript files create more HTTP requests and potentially more render blocking.
Simple fix: Combine multiple CSS files into one file and multiple JavaScript files into one file where appropriate. Then minify these files to reduce their size and loading time.
The browser typically discovers render blocking resources only after parsing HTML, adding extra time.
Simple fix: If your server supports HTTP/2, implement server push to proactively send critical CSS and JavaScript files to the browser before it even requests them.
Manually identifying critical CSS can be challenging and time-consuming.
Simple fix: Use build tools or plugins that automatically extract and inline critical CSS. For WordPress sites, plugins like Autoptimize or WP Rocket can handle this for you.
What's happening: You're loading entire CSS frameworks like Bootstrap or Foundation, even though you only use a small portion of their styles for above-the-fold content.
Simple solution: Extract only the styles you need for initial rendering, or switch to a more lightweight CSS approach like Tailwind's PurgeCSS or a custom minimal stylesheet.
What's happening: Third-party widgets (like social media buttons, chat tools, or analytics) are loading their resources in a render-blocking way.
Simple solution: Move third-party scripts to the end of the body or load them asynchronously. For CSS from third parties, either defer it or inline only the critical portions needed for above-the-fold content.
What's happening: Analytics scripts placed in the document head are delaying your page from rendering, even though they don't affect the visual display.
Simple solution: Most analytics platforms support asynchronous loading. Update your analytics implementation to use the async pattern recommended by your provider.
What's happening: Several web font files are causing the "invisible text" problem while they load, effectively blocking content visibility.
Simple solution: Implement font-display: swap, preload only the most critical font weights and styles, and consider using system fonts for better performance.
Render blocking resources directly impact several key performance metrics:
| Performance Metric | How Render Blocking Resources Affect It |
|---|---|
| First Contentful Paint (FCP) | Render blocking resources directly delay FCP, as the browser can't paint any content until they're processed |
| Largest Contentful Paint (LCP) | Since LCP can't happen before FCP, render blocking resources create a minimum delay for your most important content |
| Speed Index | By delaying the start of rendering, render blocking resources significantly impact Speed Index, which measures visual loading progression |
| Time to Interactive (TTI) | JavaScript that blocks rendering often also delays interactivity, pushing back TTI |
| Cumulative Layout Shift (CLS) | Properly loading CSS can actually reduce layout shifts by ensuring styles are available before content displays |
Optimizing render blocking resources is one of the most effective ways to improve these "early-stage" metrics that affect the initial user experience with your website.
Companies that have prioritized optimizing their render blocking resources have seen impressive results:
These examples demonstrate that optimizing render blocking resources directly impacts how quickly users can see and interact with your content, which translates to improved engagement and business results.
Render blocking resources are like keeping the curtain closed at a theater even though the stage is set and the actors are ready—they prevent your audience from seeing what you've prepared, creating an unnecessary delay that tests their patience.
The key principle to remember is that not all resources need to block rendering. By identifying what's truly essential for the initial view and optimizing how resources load, you can dramatically reduce the blank screen time your visitors experience and get your content in front of them faster.
This doesn't mean eliminating CSS or JavaScript—it means being smarter about how and when they load. Critical resources should load quickly and efficiently, while non-critical resources should load in ways that don't delay the initial display of content.
By implementing the strategies in this article, you're essentially raising the curtain sooner, allowing visitors to see your content while the final touches happen behind the scenes. This creates a more engaging, professional experience that respects your visitors' time and keeps them on your site longer.
Greadme's easy-to-use tools can help you identify render blocking resources on your website and provide simple, step-by-step instructions to optimize them—even if you're not technically minded.
Get Rid of Render Blocking Today