[Technical Errata Reported] RFC7231 (6149)

RFC Errata System <rfc-editor@rfc-editor.org> Wed, 29 April 2020 11:10 UTC

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Subject: [Technical Errata Reported] RFC7231 (6149)
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The following errata report has been submitted for RFC7231,
"Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content".

You may review the report below and at:

Type: Technical
Reported by: Alan Egerton <eggyal@gmail.com>

Section: 5.3.2

Original Text
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
that matches the type.  For example,

  Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
          text/html;level=2;q=0.4, */*;q=0.5

would cause the following values to be associated:

| Media Type        | Quality Value |
| text/html;level=1 | 1             |
| text/html         | 0.7           |
| text/plain        | 0.3           |
| image/jpeg        | 0.5           |
| text/html;level=2 | 0.4           |
| text/html;level=3 | 0.7           |

Corrected Text
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
that matches the type.  For example,

  Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
          text/plain;format=fixed;q=0.4, */*;q=0.5

would cause the following values to be associated:

| Media Type               | Quality Value |
| text/plain;format=flowed | 1             |
| text/plain               | 0.7           |
| text/html                | 0.3           |
| image/jpeg               | 0.5           |
| text/plain;format=fixed  | 0.4           |
| text/plain;delsp=yes     | 0.7           |

The optional "level" parameter of media type text/html was removed by informational RFC 2854 (The 'text/html' Media Type), [section 2](https://tools.ietf.org/html/rfc2854#section-2) of which states:

> Note that [HTML20] included an optional "level" parameter; in
> practice, this parameter was never used and has been removed from
> this specification.

More formally, [the current IANA registration of the text/html media type](https://www.iana.org/assignments/media-types/text/html), which is taken directly from [section 16.1 of the HTML specification](https://html.spec.whatwg.org/multipage/iana.html#text/html), does not include a "level" parameter.

Whilst the example is non-normative, it has given rise to misleading information—e.g. in the [MDN Web Docs glossary definition of "quality values"](https://developer.mozilla.org/en-US/docs/Glossary/quality_values)lues), which states:

> Some syntax, like the one of Accept, allow additional specifiers
> like text/html;level=1. These increase the specificity of the value.
> Their use is extremely rare.

This erratum is currently posted as "Reported". If necessary, please
use "Reply All" to discuss whether it should be verified or
rejected. When a decision is reached, the verifying party  
can log in to change the status and edit the report, if necessary. 

RFC7231 (draft-ietf-httpbis-p2-semantics-26)
Title               : Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
Publication Date    : June 2014
Author(s)           : R. Fielding, Ed., J. Reschke, Ed.
Category            : PROPOSED STANDARD
Source              : Hypertext Transfer Protocol Bis APP
Area                : Applications
Stream              : IETF
Verifying Party     : IESG