HTML language attribuut staat in grote letters.

[rtfm]

Het HTML element heeft language attribuut NL.
Op zich niet fout, maar het hoort eigenlijk in kleine letters.
NL is de de Regio en nl is de taal.

lang="fr-CA" Frans gesproken in Canada
lang="nl" Taal is Nederlands.

Meer info hier: http://www.w3.org/International/articles/language-tags/

[crisp]

Volgens http://tools.ietf.org/html/bcp47:

At all times, language tags and their subtags, including private use
and extensions, are to be treated as case insensitive: there exist
conventions for the capitalization of some of the subtags, but these
MUST NOT be taken to carry meaning.