The language-dependent text-transform casing rules (bug 231162) use exact language matches rather than using language tag matching as described in http://tools.ietf.org/html/bcp47 . The relevant code checks are pretty much every occurrence of "mLanguage" in layout/generic/nsTextRunTransformations.cpp , which is just doing atom equality checks on mLanguage. This was raised in http://css.2040035.n2.nabble.com/non-English-characters-omit-accents-when-using-text-transform-uppercase-tp7580157p7581645.html pointing to the testcase http://www.ispania.gr/tests.html .