Is the "data" attribute a standard attribute of the XHTML Strict Doctype?

Question

Can't find this info ... looks like its HTML-5ish

Answer 1

If you're talking about attributes with the data- prefix, that's indeed HTML5 only. If you want to use them in XHTML, you'd need XHTML5.

There is also an attribute whose name is just data. It's for <object> elements and works like the src attribute on images. It's part of HTML4 and XHTML1 in both Strict and Transitional doctypes.