Language information declared using the lang or xml:lang attribute is inherited by all contained elements. This means that declaring language information in the html element sets the default language for the whole document. Note, also, that this kind of declaration overrides any conflicting declaration in the HTTP header.

You can only specify a single language per element using language attributes. For this reason, this approach is not well suited to declaring primary languages where multiple languages may be involved. On the other hand, the restriction to one and only one language per element is exactly what is needed for declaring text-processing languages.

You can attach language attributes to any element to indicate that the language of text in that element is different from that of its surrounding context.

Most mainstream browsers seem to recognize the declarations made using language attributes for supported features that depend on language information.