2.1.6 [HTML] Section 8.1.2, Inheritance of language codes
The specification states:
An element inherits language code information according to the following order of precedence (highest to lowest): The lang attribute set for the element itself. The closest parent element that has the lang attribute set (i.e., the lang attribute is inherited). The HTTP "Content-Language" header (which may be configured in a server). For example: Content-Language: en-cockney- User agent default values and user preferences.
All Document Modes (All Versions)
Inherited lang attribute values are not supported.