HTTP was designed from the beginning to support multiple representations of the same resource. A user profile can be represented as JSON for a machine consumer or as HTML for a browser. A dataset can be returned as JSON or as CSV depending on what the client intends to do with it. An image can be served as JPEG or WebP based on what the browser supports. Content negotiation is the mechanism through which clients and servers agree on which representation to use — and understanding it is essential for building APIs that serve diverse consumers cleanly.

Caching is one of the highest-leverage performance improvements available to an API, and one of the least consistently implemented. A response that is cached at the right layer — in a CDN, a reverse proxy, or the client itself — eliminates a server round trip entirely. At scale, that elimination compounds: fewer database queries, lower infrastructure cost, faster responses for every consumer. The HTTP specification provides a complete, standardized caching system. Most APIs use it only partially, leaving significant headroom unrealized.