XHTML: there is no attribute "data-...

While validating an XHTML Transitional page I get 2 errors:
   there is no attribute "data-...

As I understand the reference, it is OK the have a custom data-... attribute in
XHTML, yet validator is flagging it as an error. Did I miss something?

ref:
https://www.w3.org/TR/2011/WD-html5-20110525/elements.html#embedding-custom-non-visible-data-with-the-data-attributes

address of the document I was checking:
   http://backyardengineering.com

Thanks,
John

Received on Thursday, 2 February 2017 13:40:15 UTC