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RenderBlockingResources

Render-Blocking vs. Parser-Blocking Resources​

When a browser loads a webpage, it processes the HTML, CSS, and JavaScript in a sequence. Some resources can block this process, delaying the display of content to the user.

1. Render-Blocking Resources​

Definition: These are resources that delay the browser from rendering content on the screen.

What Causes Render-Blocking?​

  • CSS Files: The browser must fully download and parse CSS before rendering the page (because styles affect layout).
  • Synchronous JavaScript (<script> without defer or async): Blocks rendering because it might modify the DOM or CSS.

How to Fix Render-Blocking?​

✅ Minify & Compress CSS/JS – Reduce file size.
✅ Use Critical CSS – Inline only the CSS needed for above-the-fold content.
✅ Defer Non-Critical CSS – Load non-essential styles asynchronously.
✅ Use async or defer for JS:

  • defer: Loads JavaScript in the background and executes it after parsing the HTML.
  • async: Loads JavaScript in parallel and executes it immediately when loaded.

2. Parser-Blocking Resources​

Definition: These block the HTML parser from continuing until they are fully processed.

What Causes Parser-Blocking?​

  • Synchronous JavaScript (<script> without defer or async).
    • The parser stops, fetches, downloads, and executes the script before continuing.

How to Fix Parser-Blocking?​

✅ Use defer for JavaScript – The parser keeps running while the script loads.
✅ Move <script> to the bottom of <body> – Ensures HTML is parsed first.
✅ Use a CDN for JavaScript Libraries – Faster retrieval speeds.

Quick Summary​

TypeBlocks Rendering?Blocks Parsing?Fixes
Render-BlockingYesNoMinify CSS, Inline Critical CSS, Defer JS
Parser-BlockingNoYesUse defer, Move scripts to bottom