Crystal Ball checks covered
RENDER_BLOCKING_CSS, RENDER_BLOCKING_JS, TOTAL_BLOCKING_TIME
What this warning means
This warning is more specific than simply having many frontend assets. It means important rendering work waits behind CSS or JavaScript that the browser must process first.
- Some CSS is necessary to prevent an unstyled flash.
- The problem is noncritical code that blocks the first screen.
Common causes
Common causes include large framework bundles, plugin CSS, unused design-system files, font stylesheets, analytics tags loaded too early, animation scripts, and third-party widgets inserted in the head.
- CSS in the head is normal, but it should be lean.
- Scripts should usually be deferred unless they are required before rendering.
How to fix it
Separate critical and noncritical resources. Keep the CSS needed for the first screen small, defer JavaScript that is not needed immediately, and remove code that no longer supports live components.
- Add defer to noncritical scripts.
- Load widget scripts only where the widget appears.
- Inline only genuinely critical CSS when the site architecture supports it.
- Remove unused libraries instead of optimizing around them.
How to verify
Retest after each change and check that menus, forms, galleries, and tracking still work. Performance fixes are only useful if the page still behaves correctly.
- Review Lighthouse render-blocking diagnostics.
- Check Total Blocking Time.
- Open the page in a fresh browser profile.
- Use Crystal Ball to confirm no new technical issues appeared.
FAQ
Is all CSS render-blocking bad?
No. Some CSS must load early so the page looks correct. The goal is to keep early CSS focused and move noncritical work later.
Can defer break JavaScript?
Yes. Scripts that depend on execution order or early DOM changes need testing after defer is added.
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