Definition Render-blocking resources stop in the browser's critical render path. Rendering does not start until CSSOM is built. Without defer or async, a <script> tag pauses the HTML parser.
How it works While parsing HTML, the browser hits a render-blocking resource and waits for it to download and process. CSS blocks by default. JavaScript blocks by default but turns non-blocking with defer or async. As the count of blocking resources grows, LCP time stretches.
Where you see it in Scope Trends The **Site Crawl** report extracts the render-blocking resource list per page. The **Core Web Vitals** tab orders LCP-impacting blockers by priority.
Frequently asked questions **Should all CSS load async?** No. Keep critical CSS inline; delay non-critical CSS with preload or media queries.
**Defer vs async difference?** Defer runs after DOM is ready and preserves order. Async runs immediately after download with no order guarantee.