RFC 2295 (HTTP 透過内容折衝) 6.4 Features attribute

The features attribute, for which section 5.1 defines the syntax

• "{" "features" feature-list "}"

is used in a variant description to specify how the presence or absence of particular feature tags in the user agent affects the overall quality of the variant.

• feature-list = 1%feature-list-element

•        feature-list-element = ( fpred | fpred-bag )
                                [ ";" [ "+" true-improvement  ]
                                      [ "-" false-degradation ]
                                ]

• fpred-bag = "[" 1%fpred "]"
• true-improvement = short-float
• false-degradation = short-float

Features attributes are used in variant descriptions (section 5). Variant descriptions can be transmitted in Alternates headers (section 8.3).

Examples are:

• {features !textonly [blebber !wolx] colordepth=3;+0.7}
• {features !blink;-0.5 background;+1.5 [blebber !wolx];+1.4-0.8}

The default value for the true-improvement is 1. The default value for the false-degradation is 0, or 1 if a true-improvement value is given.

A user agent SHOULD, and a remote variant selection algorithm MUST compute the quality degradation factor associated with the features attribute by multiplying all quality degradation factors of the elements of the feature-list. Note that the result can be a factor greater than 1.

feature-list の要素の品質減衰因子すべてを掛け合わせることによる features 属性に関連付けられた品質減衰因子の計算を利用者エージェントは行うべきですし、 遠隔変種選択算法は計算しなければなりません。

A feature list element yields its true-improvement factor if the corresponding feature predicate is true, or if at least one element of the corresponding fpred-bag is true. The element yields its false-degradation factor otherwise.

特徴目録要素は、対応する特徴述語が真であるか、 対応する fpred-bag の少なくても1つが真である場合には true-improvement 因子を与えます。 それ以外の時には要素は false-degradation 因子を与えます。