Are Custom Attributes OK in XHTML

Question

I understand that according to the HTML specification, it's invalid to add custom attributes to elements. Is this also invalid with XHTML?

I thought XHTML was part of the XML family, and as such was extensible. Being extensible, isn't it ok to use custom attributes?

Dave

Answer 1

custom attributes won't be considered valid by the standard W3C validators. You can define your own document type definition (DTD) though. See http://www.alistapart.com/articles/customdtd/ for more information about that.

Answer 2

With the standard document type definition, you can't introduce your own custom attributes.

But, starting with HTML5 you'll be able to introduce your own attributes as long as you prefix them with data-.