Content-Length header

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

The HTTP Content-Length header indicates the size, in bytes, of the message body sent to the recipient.

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.

In HTTP/1.0, it is required.
In HTTP/1.1, it could be replaced with Transfer-Encoding: chunked for responses sent out in parts as its size is calculated.
In HTTP/2, Content-Length is redundant, because the content length may be inferred from DATA frames. It may still be included for backwards compatibility.