mimecuvalo/html5.vim — comment on commit 5becce035c7d1e28b49a31ea2ba20b127d466d54
File:  line 0
By yangg:

A custom data attribute is an attribute in no namespace whose name starts with the string "data-", has at least one character after the hyphen, is XML-compatible, and **contains no characters in the range U+0041 to U+005A** (LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).

http://dev.w3.org/html5/spec/elements.html#embedding-custom-non-visible-data-with-the-data-attributes