A CORS error appears in the browser console as if it's a client-side problem — but it's actually caused by missing response headers from the server, which is why "it works in curl/Postman but fails in the browser" is the classic CORS symptom. Here's same-origin policy, the key CORS headers, simple vs preflighted requests, and a systematic debugging approach.
Content-hashed filenames with max-age=31536000 can eliminate network requests for returning users. Here's the complete HTTP caching strategy: Cache-Control directives in depth, ETags and conditional requests, the Vary header pitfall, CDN cache busting, and a practical caching strategy by resource type.
Security headers most sites are missing, what HSTS, CSP, and X-Frame-Options actually prevent, how Cache-Control directives work, and which headers reveal server information you should hide in production.
Learn what HTTP response headers are, what the key headers (Cache-Control, HSTS, CSP, CORS, X-Frame-Options) mean, and how to use a free HTTP headers checker for security audits and debugging.
