XHTML 1.1 is the latest official (W3C-approved) version of HTML, and it was issued as a W3C Recommendation on May 31, 2001. The specification can be found at
XHTML 1.1 documents must have the following DOCTYPE declaration:
<!DOCTYPE html PUBLIC "-//W3C/DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
The namespace for XHTML 1.1 is the XHTML namespace, which can be found here:
XHTML 1.1 is a reformulation of XHTML 1.0 Strict in modularized XHTML. Deprecated modules were dropped entirely, and frames are not available either because they're not included in XHTML 1.0 Strict.
The modules used to create XHTML 1.1 are ...