right doctype for RDFa breadcrumbs navigation and validator

Question

I need to make my mind clear about HTML doctypes. in this page: http://kovo.intl.uk.to I add breadcrumbs navigation using RDFa. but then page was no more valid. I googled and I

Answer 1

From Wikipedia:

XHTML 1.0 is "a reformulation of the three HTML 4 document types as applications of XML 1.0"

So you are fine, it's still an equivalent of HTML 4:

 There are three formal DTDs for XHTML 1.0, corresponding to the three different versions of HTML 4.01:

- XHTML 1.0 Strict is the XML equivalent to strict HTML 4.01, and includes elements and attributes that have not been marked deprecated in the HTML 4.01 specification. As of May 25, 2011, XHTML 1.0 Strict is the document type used for the homepage of the website of the World Wide Web Consortium.
- XHTML 1.0 Transitional is the XML equivalent of HTML 4.01 Transitional, and includes the presentational elements (such as center, font and strike) excluded from the strict version.
- XHTML 1.0 Frameset is the XML equivalent of HTML 4.01 Frameset, and allows for the definition of frameset documents—a common Web feature in the late 1990s.

XHTML+RDFa from Wikipedia:

XHTML+RDFa is one of the techniques used to develop Semantic Web content by embedding rich semantic markup. Version 1.1 of the language is a superset of XHTML 1.1