Understanding HTTP Headers When Viewing Source

Every HTTP response includes headers — small pieces of metadata that describe the content and the rules for using it. When viewing source, headers explain why a page loads (or doesn’t) and what format you’re looking at.

Key headers to know

• Status: 200 OK, 301 Moved Permanently, 404 Not Found, etc.
• Content-Type: text/html; charset=utf-8 tells you it’s HTML; JSON/XML/images will differ.
• Content-Encoding: gzip or br (Brotli) compression.
• Cache-Control and ETag: caching behaviour.
• Content-Security-Policy (CSP): where content may load/execute.
• X-Robots-Tag or meta robots: indexing/crawling hints.

Using a header inspector

A header inspector shows status, size, type, and selected security directives above the source. It helps diagnose blank pages, mismatched encodings, or non-HTML responses at a glance.

Examples

• JSON API returning application/json — you’ll see JSON, not HTML.
• SPA returning minimal HTML — the “real” content is rendered on the client.
• Redirect chains — watch for 301/302 hops before the final HTML.

Inspect some headers →