1. Why Google Needs Access to JS, CSS, and Fonts
Googlebot uses a headless browser (Chrome-based) to render pages. It evaluates:
- Visual layout
- Mobile usability
- Content loaded via JavaScript
- Core Web Vitals
If CSS/JS is blocked, Google may misinterpret the page structure, flag it as non-mobile-friendly, or fail to render content.
2. What Happens When You Block Resources
Blocked CSS → Layout breaks, mobile usability fails
Blocked JS → Dynamic content doesn’t load (e.g., product listings, reviews)
Blocked Fonts → CLS issues may trigger Core Web Vitals penalties
In Search Console, affected pages often show:
"Googlebot cannot access resources on this page"
3. Common Mistakes in robots.txt
Disallow: /assets/
Disallow: /js/
Disallow: /css/
Disallow: *.woff
These rules block essential render resources. Even worse if applied globally across CMS themes or frontend frameworks.
4. How to Audit and Fix
Search Console → Pages → Blocked by robots.txt
Use URL Inspection Tool to see rendered HTML vs. source HTML
Crawl with Screaming Frog (Rendering Mode: JavaScript) to simulate what Google sees
Remove or refine disallow rules.
Use our robots.txt validator to check if your robots file is correct.
5. When Blocking Might Be Acceptable
3rd-party analytics or A/B testing scripts
Unused or legacy font packs
Resource folders unrelated to the front-end
But always test whether removing them affects rendered content or layout.
Conclusion
Blocking JavaScript, CSS, or fonts in robots.txt can cripple how Google sees your site. If these resources affect layout or content delivery, they must be crawlable. Audit renderability regularly—and stop hiding your site’s real experience from search engines.