2.1.7 [HTML] Section 8.1.3, Interpretation of language codes

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V0007:

The specification states:

 In the context of HTML, a language code should be interpreted by user agents as a 
 hierarchy of tokens rather than a single token. When a user agent adjusts rendering 
 according to language information (say, by comparing style sheet language codes and 
 lang values), it should always favor an exact match, but should also consider 
 matching primary codes to be sufficient. Thus, if the lang attribute value of "en-
 US" is set for the HTML element, a user agent should prefer style information that 
 matches "en-US" first, then the more general value "en".

Quirks Mode and IE7 Mode (All Versions)

The pseudo-class :lang is not supported.

IE8 Mode and IE9 Mode (All Versions)

The pseudo-class :lang is applied in source order. More exact matches are not favored over less exact ones.