How to Detect Render-Blocking JavaScript or CSS Files
To detect render-blocking JS or CSS, use tools like Google PageSpeed Insights, Lighthouse, or Chrome DevTools to analyze your page load performance and identify files that delay First Contentful Paint (FCP).
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Key Takaways
Frequently Asked Questions
What are render-blocking resources?
They delay First Contentful Paint (FCP) and Largest Contentful Paint (LCP), which are key Core Web Vitals. Poor performance can harm rankings.
How do render-blocking files affect SEO?
They delay First Contentful Paint (FCP) and Largest Contentful Paint (LCP), which are key Core Web Vitals. Poor performance can harm rankings.
What tools help detect render-blocking files?
Google PageSpeed Insights, Lighthouse, Chrome DevTools, and WebPageTest are among the best tools to identify and analyze such files.
How do I fix render-blocking JS?
Add 'async' or 'defer' to script tags, split JS bundles, and load only what's needed for above-the-fold content.
Should all CSS be deferred?
No, critical CSS should be inlined or loaded early. Only non-critical CSS (e.g., below-the-fold content) should be deferred.
Step by Step Plan
Run a PageSpeed Insights Test
Go to PageSpeed Insights, enter your URL, and review the 'Eliminate render-blocking resources' section for flagged JS/CSS files.
Analyze with Chrome DevTools
Open DevTools (F12), go to the 'Performance' tab, record a page load, and identify long loading scripts that occur before paint events.
Check the Critical Rendering Path
Focus on scripts/styles that load in the head and delay visual elements—these are considered render-blocking.
Check the Critical Rendering Path
Use tools like Coverage tab in DevTools or CSS/JS analyzers to detect unused styles and non-critical resources.
Look for Optimization Opportunities
Consider deferring non-critical JS or using media queries/split CSS files for faster, prioritized loading.
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