Definition
Render-blocking resources are CSS and JavaScript files that must be downloaded and processed by the browser before it can display page content. By default, CSS is render-blocking (the browser waits for the complete CSSOM before painting) and synchronous JavaScript blocks DOM parsing. These resources delay First Contentful Paint (FCP) and Largest Contentful Paint (LCP), degrading Core Web Vitals. Solutions include: loading non-critical CSS asynchronously (media='print' onload), inlining critical CSS, using defer or async on script tags, loading third-party scripts at the end of the page or via a tag manager with deferral, and using preload for critical resources. Google PageSpeed Insights specifically flags render-blocking resources as an issue to fix.
Key Points
- CSS is render-blocking by default; use critical inlining + async loading
- Synchronous JavaScript blocks DOM parsing; prefer defer and async
- Google PageSpeed Insights automatically identifies render-blocking resources
Practical Examples
Deferring third-party scripts
A site loads 8 third-party scripts (analytics, chat, tracking) synchronously. After switching to defer and conditional loading, FCP improves by 1.5 seconds.
Asynchronous CSS
A site with 3 CSS files (400KB total) transforms them: critical CSS (15KB) is inlined, the 2 non-critical files are loaded with media='print' onload='this.media="all"'. LCP improves by 800ms.
Frequently Asked Questions
async downloads the script in parallel and executes it as soon as available (may interrupt parsing). defer downloads in parallel but waits until HTML parsing is complete before executing, and respects script order. defer is generally preferred for scripts that depend on the DOM.
The recommended technique is: <link rel='stylesheet' href='style.css' media='print' onload='this.media="all"'>. This loads CSS in the background then applies it once downloaded. Combine with critical CSS inlining for a fast first render.
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Last updated: 2026-02-07