2.1.7 [HTML] Section 8.1.3, Interpretation of language codes
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.