This is mainly about solving character encoding issues rather than DOCTYPE declaration for supporting three mobile carriers.

Specifying the character encoding with charset in the head tag solves character encoding problems.

■ For HTML

■ For XHTML

・About DOCTYPE Declaration for Supporting Three Mobile Carriers | ke-tai.org - InfiniteLoop

・How to Create Mobile Sites Supporting Three Carriers with XHTML | Weblancer

・Let’s Declare Character Encoding (HTML) - Web Standards Promotion Project

That’s all from the Gemba.