render-blocking resources
Also called: render blocking, render-blocking CSS, render-blocking JavaScript
Render-blocking resources are CSS and JavaScript files a browser must download and process before it can paint any page content. They delay First Contentful Paint and Largest Contentful Paint, slowing how fast users (and Core Web Vitals) see your page.
What blocks rendering
By default a <link rel="stylesheet"> in the <head> is render-blocking: the browser will not paint until it builds the CSSOM, so a page never flashes unstyled. A synchronous <script> (no async or defer) is worse, it is parser-blocking, stopping HTML parsing entirely because the script could rewrite the DOM or CSSOM. Lighthouse flags both, now grouped under its “render-blocking requests” insight.
The fixes are three moves:
- Inline the critical above-the-fold CSS in a
<style>block and load the rest asynchronously. - Add
deferorasyncto non-critical scripts so they stop holding up the parser. - Split stylesheets by
mediaso the browser only blocks on the ones matching the device.
<link rel="stylesheet" href="print.css" media="print">
<script src="analytics.js" defer></script>Use the Chrome DevTools Coverage tab to see how much CSS and JS a page actually uses on first load. Unused bytes sitting in the critical path are pure delay. The point is the initial paint, not total page weight: a 200KB script loaded with defer does not block rendering, while a 5KB blocking one in the head does. Strip, defer, or inline until nothing unnecessary stands between the browser and the first paint.
How it affects your traffic
Render-blocking resources cost you twice. They push out First Contentful Paint and Largest Contentful Paint, and LCP is a Core Web Vitals metric inside Google's page experience signals. Slow first paint also drives up bounce on mobile, where blocking bytes hurt most. A technical SEO pass finds the exact files stalling your critical path and moves them off it, so pages paint sooner and Core Web Vitals stay in the green.
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