MIME Type Validation

Incorrect MIME types cause browsers to block stylesheets and scripts (MIME sniffing is disabled by X-Content-Type-Options: nosniff), breaking page rendering and signalling poor site quality to crawlers.

Quick Reference

• The Content-Type header must match the actual file type being served
• HTML pages must be served as text/html; charset=utf-8
• CSS files must be served as text/css and JavaScript as text/javascript
• Incorrect MIME types cause browsers to block or mishandle resources

Check

For key pages and their linked resources, inspect the HTTP Content-Type response header. Verify HTML responses use text/html; charset=utf-8, CSS uses text/css, JS uses text/javascript, and images use image/jpeg, image/png, etc.

Fix

Update your server configuration to serve each file type with its correct MIME type. Set X-Content-Type-Options: nosniff to prevent browsers from guessing content types. In Next.js and similar frameworks, configure headers in next.config.js.