[selectors-4] `language-range` is missing its definition

[aphillips]

7.2. The Language Pseudo-class: :lang()
https://www.w3.org/TR/2026/WD-selectors-4-20260122/#language-range

Each language range in :lang() must be a valid CSS or . (Thus language ranges containing asterisks, for example, must be either correctly escaped or quoted as strings, e.g. :lang(*-Latn) or :lang("*-Latn").)

The <dfn> for "language range" has no definition and really it should provide one. Either it should be linked to the one in I18N Glossary (here), to the definition of an extended language range in BCP47 (in the RFC4647 part), or it should provide one locally.

[svgeesus]

Certainly a definition should be linked to.

The I18n glossary entry seems imprecise ("similar to"):

A string similar in structure to a language tag that is used for "identifying sets of language tags that share specific attributes".

[svgeesus]

The definition from BCP47 looks crisper, with a defined grammar:

Occasionally, users will wish to select a set of language tags based
on the presence of specific subtags. An "extended language range"
describes a user's language preference as an ordered sequence of
subtags. For example, a user might wish to select all language tags
that contain the region subtag 'CH' (Switzerland). Extended language
ranges are useful for specifying a particular sequence of subtags
that appear in the set of matching tags without having to specify all
of the intervening subtags.

An extended language range can be represented by the following ABNF:

extended-language-range = (1*8ALPHA / "*")
*("-" (1*8alphanum / "*"))

The wildcard subtag '*' can occur in any position in the extended
language range, where it matches any sequence of subtags that might
occur in that position in a language tag. However, wildcards outside
the first position are ignored by Extended Filtering (see Section
3.2.2). The use or absence of one or more wildcards cannot be taken
to imply that a certain number of subtags will appear in the matching
set of language tags.

@fantasai @tabatkins do you agree that a link to the BCP47 definition of Extended Language Range should be added?

[svgeesus]

@aphillips pull request to fix this, could you review?

• [selectors-4] Defined language range  #13748