docs (content-length) : Add HTTP version comparison

[Julien-Dochez]

Description

Added a comparison table clarifying 'Content-Length' header behavior differences between HTTP/1.1 and HTTP/2.

Motivation

I made this simple change because it helps resolve confusion in a simple way. Developers may use this information to avoid unecessary header usage.

Additional details

https://httpwg.org/specs/rfc9113.html#rfc.section.8.1

Related issues and pull requests

Fixes #36681

[Josh-Cena]

Just want to point out that this is very low information density and very un-MDN because you could have been a single sentence in the intro:

Suggested change

	The handling of `Content-Length` differs between HTTP versions:
	| Feature | HTTP/1.1 | HTTP/2 |
	| --------------- | -------------------------------- | ------------------------- |
	| Required | Yes (for persistent connections) | Optional |
	| Primary purpose | Frame body size | Backward compatibility |
	| Alternatives | `Transfer-Encoding` | Inferred from DATA frames |
	`Content-Length` was required in HTTP/1.1 for persistent connections to indicate the frame body size. It became optional in HTTP/2 because the size can now be inferred from DATA frames, and this header is only kept for compatibility.

[hamishwillee]

@Josh-Cena I was writing my response at the same time as you - #40534 (comment) - took me a lot longer

Similar feeling except that I thought (something like) this was useful to add in the introduction too:

Content-Length is limited in that the message size must be known up front, before sending the headers, which is a problem when content is dynamically generated or streamed. Transfer-Encoding: chunked allows the content to be sent out in parts as its size is calculated.

[hamishwillee]

So maybe we combine those two approaches in the intro?

[Josh-Cena]

Yes let's go with the wording you would prefer; I'm just paraphrasing here and you would know better

[hamishwillee]

Cool. We agree on the main point of "MDN style". I lean towards #40534 (comment)

[github-actions]

Preview URLs

• /en-US/docs/Web/HTTP/Reference/Headers/Content-Length

Flaws (3)

URL: /en-US/docs/Web/HTTP/Reference/Headers/Content-Length
Title: Content-Length header
Flaw count: 3

• unknown:
  • No generic content config found
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(comment last updated: 2025-07-28 03:48:32)

[Josh-Cena]

FWIW I don't like ad-hoc H2 headings, would prefer if you change it to "description" or just put this in the intro.

[Josh-Cena]

Brian is ooo for a few weeks. Happy to give you a rubberstamp.

[hamishwillee]

Thanks @Josh-Cena and @Julien-Dochez .