HTTP Semantics (RFC 9110)

1. Terminology and core concepts

   Resources, representations, clients, servers, intermediaries (proxies, gateways, tunnels) and caches.

2. Fields

   Header and trailer field syntax, field order and combining multiple field lines.

3. Representation data and metadata

   Content-Type, Content-Encoding, Content-Language and the Content-Location header.

4. Validators: ETag and Last-Modified

   Strong and weak validators used for caching and conditional requests.

5. Methods and their properties

   GET, HEAD, POST, PUT, DELETE, CONNECT, OPTIONS, TRACE with safe and idempotent semantics.

6. Safe and idempotent methods

   Which methods are safe, idempotent or cacheable and why it matters for retries.

7. Content negotiation

   Proactive negotiation with Accept headers and Vary, and reactive negotiation.

8. Conditional requests

   If-Match, If-None-Match, If-Modified-Since and precondition evaluation order.

9. Range requests

   Range, Content-Range, 206 Partial Content and If-Range.

10. Status codes

    Meaning of 1xx, 2xx, 3xx, 4xx and 5xx status codes including redirects.

11. HTTP authentication framework

    WWW-Authenticate challenges, Authorization credentials and the Basic scheme framework.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.