MDN: HTTP

What it is

MDN's HTTP section is a set of guides and reference pages about the protocol behind every web request. It starts with an overview of how a client and server exchange messages, then covers methods such as GET and POST, response status codes, headers, cookies, caching, CORS, redirects and authentication. Each method, status code and header has its own short reference page with examples.

Who it is for

Anyone who calls APIs without fully understanding what is happening underneath: students starting backend work, frontend developers who want to understand fetch errors, and people building automations in tools like n8n, Make or Zapier. If a workflow has ever failed with a 401 or a 429 and you were not sure why, this is the place to fix that gap.

How to use it

You do not need to read it cover to cover. A good first pass takes two or three hours:

• Read the overview and the page on HTTP messages, so you know what a request line, headers and a body are.
• Read the request methods page and skim each method.
• Read the status codes page and learn the families: 2xx success, 3xx redirects, 4xx client errors and 5xx server errors.
• Look up the headers you will meet most: Content-Type, Authorization, Accept and Retry-After.

After that, treat it as a reference. When an API returns something unfamiliar, look up that exact code or header.

Is it worth it?

Yes, and it is free. Most API documentation assumes you already know HTTP, and most automation tools hide it behind friendly forms. Knowing it lets you read any API's docs, debug failed requests from the status code, and decide which errors are worth retrying. The content is written by Mozilla and community contributors and is maintained as an open project on GitHub, so it tracks current standards.

What it does not cover

It explains the protocol, not particular APIs. For designing or calling a specific service you still need that service's own documentation, and for practice you need a tool such as Postman or curl to send real requests. It also does not teach a programming language.

Common questions

Do I need to know all the headers? No. A handful cover most daily work; look the rest up when they appear.

Is it too technical for non-developers? The overview pages are readable by anyone comfortable with basic web concepts. The reference pages are denser but short.

How is it different from the main MDN Web Docs site? It is one section of MDN, focused only on HTTP, so it is a better starting point than the whole site when your goal is working with APIs.