How to document an HTTP header

The HTTP headers reference on MDN Web Docs documents HTTP header fields. These are components of the header section of request and response messages in the Hypertext Transfer Protocol (HTTP). They define the operating parameters of an HTTP transaction. This article explains how to create a new reference page for an HTTP header.

You will need to know or be able to dive into some HTTP.

Step 1 – Determine the HTTP header to document

There are many HTTP headers defined in various IETF standards.
IANA maintains a registry of headers and Wikipedia lists the known header fields, but not all are relevant to web developers or are part of an official standard.
If there are any red links on the current HTTP headers reference overview page, these headers are a good choice to document.
If in doubt, ask the MDN Web Docs team whether or not it makes sense to write about the header you have chosen.

Step 2 – Check the existing HTTP header pages

Existing HTTP headers are documented here.
There are different header categories: Request header, Response header, and Representation header.
Find the category of the header you are about to document (note that some headers can be both request and response headers, depending on the context).
Go to an existing header reference page that has the same category.

Step 3 – Create the HTTP header page

All header pages live under this tree: /docs/Web/HTTP/Headers/
To create a new page, see the instructions in our how to create a page guide.

Step 4 – Write the content

Either start from our template HTTP header page or use a copied structure from one of the existing HTTP header documents that you found in step 2. It's your choice.
Write about the new HTTP header.
Make sure you have these sections:
- Introductory text where the first sentence mentions the header name (bold) and summarizes its purpose.
- Information box containing at least the header type and if the header is a Forbidden header name.
- A syntax box containing all possible directives/parameters/values of the HTTP header.
- A section that explains these directives/values.
- An example section that contains a practical use case for this header or shows where and how it occurs usually.
- A specification section listing relevant RFC standard documents.
- A "See also" section listing relevant resources.

Step 5 – Add browser compatibility information

If you have looked at other HTTP header pages, you will see that there is a {{Compat}} macro that will fill in a browser table for you.
The compatibility table page is generated from structured data. If you'd like to contribute to the data, please check out the instructions at https://github.com/mdn/browser-compat-data/blob/main/README.md and send us a pull request.

Step 6 – Update the HTTP headers list

Make sure your header is listed in an appropriate category on the HTTP headers reference overview page.

Step 7 – Get the content reviewed

After you've created the header page, submit it as a pull request. A member of our review team will be assigned automatically to review your page.