HTTP Accept "level"?

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Solution

...A Couple Years Later

The user TomWardrop points out that (from comments to this answer):

It use to be part of the text/html media type specification to indicate what version of HTML you wanted, but is wasn't used much (if at all), so it was dropped. Refer to ietf.org/rfc/rfc2854.txt.

Excerpt from the RFC:

Note that [HTML20] included an optional "level" parameter; in practice, this parameter was never used and has been removed from this specification. [HTML30] also suggested a "version" parameter; in practice, this parameter also was never used and has been removed from this specification.

This appears to be the best answer so far.

I am leaving the original answer below for posterity.

Summary

It appears from looking through RFC-2616 (Request For Comments) over at IETF and W3C, as well as websites elsewhere on the internet, that the `level` extension is not very well documented or explained. It also does not appear to be in use in headers by anyone, suggesting that it can probably be ignored.

The RFCs

In the RFCs the `level` parameter is seen in a few examples, but it is never mentioned or clearly expressed exactly what role it plays.

`level` is used in an example about precedence:

Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example,

    Accept: text/*, text/html, text/html;level=1, */*

have the following precedence:

    1) text/html;level=1
    2) text/html
    3) text/*
    4) */*

Source: IEFT RFC-2616 p.100 and W3C RFC2616 section "14.1 Accept"

Seeing the difference between how the types are ordered in the two examples, it looks like `text/html;level=1` has greater precedence than `text/html`, meaning that the `level` extension must give it that precedence. The last two are obviously ordered further, according to declining specificity.

Now this brings up the quality factor, `q`. It is explained quite well in the RFCs. It can be anything between `0` and `1`. The bigger the value the more precedence the type has. The RFC has an example using both `q` and `level`:

The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that 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:

    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

Source: IEFT RFC-2616 p.100 and W3C RFC2616 section "14.1 Accept"

From this it appears that `level`'s value is used in decreasing precedence (1 has highest precedence, 2 second highest, etc.). That makes sense, but when taken together with the `Accept:` header it makes no sense at all:

First, the `q` parameters have magically disappeared from the types, in the associations (below). It is like the author assumed that it is obvious why they have been omitted, but forgot to tell us why it is obvious.

Second, the types in the `Accept:` header, and the types shown in the associations (below), are not the same types. E.g the `image/jpeg` type is never mentioned in the header, and the `text/*` type is missing from the associations.

I am at a loss to explain what it all means.

Zend Framework Discussion

On an answer to a question asking about the `level` parameter there is some interesting stuff.

`q` and `level` compliment each other

[...] The q-factor is the one most looked at. However, you can also specify a "level", and these CAN also act like priorities, but operate in the order of decreasing precedence (i.e., level 1 is higher priority than level 2). The examples they have in the spec are contrived, and, tbh, confusing at best [...]

The `q` parameter is what you should use, and the `level` parameter can be used. Okay, but it still does not clear up exactly what it does, and how it is supposed to affect the priorities of types.

Used for supported type/spec version

Another documented use case for the "level" is to indicate the _version_ of the type. As an example, a "level 1" might indicate the first version of that spec available. In such cases, it wouldn't be a priority, but instead a _descriptor_ [...]

So, a way of indicating what version of the type is supported. Now, this actually makes more sense:

text/html;level=5;q=1
text/html;level=4.01;q=0.9
; Etc.

Somehow I doubt `level` was supposed to be used for this, however. If it was then it would probably have been called something more descriptive, such as `version`, or simply `v` (like `q`).

Conflicting meanings

Unfortunately, the "level" selector has conflicting meanings, so I'm not 100% sure we should support it by default; "q", on the other hand, is well documented.

Yeah. Conflicting meanings and not very well documented. The `level` property has very little going for it.

Other stuff I found

Looking for questions/answers elsewhere on the Internet I found:

- A seriously old-school document that does not mention `level` at all. It actually mentions two other ones, `mxs` and `mxb`.

- A Moz Dev page listing common `Accept` headers from various browsers. Nowhere is `level` used. In fact, I have never seen it used outside the RFCs.

Conclusion

In conclusion I think it is safe to say that `level` is a waste of time. It is poorly documented, not used much in practice (if used at all), and it is more confusing than it is worth.

Problem

Original source