ISSUE-240: Remove requirement of validity to self-declared DTD [mobileOK Basic tests]

ISSUE-240: Remove requirement of validity to self-declared DTD [mobileOK Basic tests]

http://www.w3.org/2005/MWI/BPWG/Group/track/issues/

Raised by: Dominique Hazaël-Massieux
On product: mobileOK Basic tests

validity to self-declared DTD is problematic for implementations:
 * it requires having SGML validation in checkers (for non XML versions of HTML)
 * it requires downloading DTDs from unknown locations when a page uses a non-standard doctype.

We could:
 * remove that test (given that we already have a fairly stringeant test of XHTML Basic validity)
 * replace it by requiring that step only when the System ID matches a well-known DTD (which would be listed in the spec)