The Accept-Features request header can be used by a user agent to give information about the presence or absence of certain features in the feature set of the current request. Servers can use this information when running a remote variant selection algorithm.

Accept-Features 要求頭は、 利用者エージェントが現在の要求の特注集合中のある特徴があるかないかについての情報を与えるために使うことが出来ます。 サーバーは、遠隔変種選択算法を動作させる時にこの情報を使うことが出来ます。

Note: the name `Accept-Features' for this header was chosen because of symmetry considerations with other Accept- headers, even though the Accept-Features header will generally not contain an exhaustive list of features which are somehow `accepted'. A more accurate name of this header would have been `Feature-Set-Info'.

注意 : Accept-Features というこの頭の名前は、 他のAccept‐頭との整合性のために選ばれましたが、 Accept-Features 頭は通常はともかく「受け入れる」 特徴の完璧な目録を含みはしません。 この頭のもっと正確な名前は Feature-Set-Info 辺りだったでしょう。

       Accept-Features = "Accept-Features" ":"
                   #( feature-expr *( ";" feature-extension ) )

       feature-expr = [ "!" ] ftag
                    | ftag ( "=" | "!=" ) tag-value
                    | ftag "=" "{" tag-value "}"
                    | "*"

       feature-extension = token [ "=" ( token | quoted-string ) ]

No feature extensions are defined in this specification. An example is:

       Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
                  paper = A4, paper!="A2", x-version=104, *

The different feature expressions have the following meaning:

ftag

ftag is present

!ftag

ftag is absent

ftag=V

ftag is present with the value V

ftag!=V

ftag is present, but not with the value V

ftag={V}

ftag is present with the value V, and not with any other values

*

the expressions in this header do not fully describe the feature set: feature tags not mentioned in this header may also be present, and, except for the case ftag={V}, tags may be present with more values than mentioned.

各特徴式は次の意味を持ちます。

ftag

ftag がある

!ftag

ftag がない

ftag=V

ftag は値 V と共にある

ftag!=V

ftag はあるが値 V と共にはない

ftag={V}

ftag は値 V と共にあり、他のいかなる値とも共にはない

*

この頭の式は特徴集合を完全に記述していない。 この頭に示されていない特徴札はあるかもしれないし、 ftag={V} の場合を除き、示されている値以外もあるかもしれない。

Absence of the Accept-Features header in a request is equivalent to the inclusion of

• Accept-Features: *

By using the Accept-Features header, a remote variant selection algorithm can sometimes determine the truth value of a feature predicate on behalf of the user agent. For example, with the header

       Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
                  paper = A4, paper!="A2", x-version=104, *

the algorithm can determine that the following predicates are true:

       blex, colordepth=[4-], colordepth!=6, colordepth, !screenwidth,
       paper=A4, colordepth=[4-6]

and that the following predicates are false:

       !blex, blebber, colordepth=6, colordepth=foo, !colordepth,
       screenwidth, screenwidth=640, screenwidth!=640,

but the truth value of the following predicates cannot be determined:

       UA-media=stationary, UA-media!=screen, paper!=a0,
       x-version=[100-300], x-version=[200-300], x-version=99,
       UA-media=screen, paper=A0, paper=a4, x-version=[100-199], wuxta