From http://lists.w3.org/Archives/Public/public-comments-wcag20/2006May/0119.html
text
sequence of characters[INS: [.] :INS] Note: Characters are those included in the Unicode/ISO/IEC 106464 repertoire.
One may use only characters in Unicode. Given that several scripts are [8]unencoded in Unicode, this may present a problem. Some East Asian languages are more robustly published with legacy encodings even if that is ""improper."" I repeatedly tried to explain to the Working Group that all that matters is a defined and understandable character encoding.
We have changed the definition of text and non-text so that it is clear that we don't preclude the use of non-Unicode encodings as long as AT can handle them (i.e. they can be 'programmatically determined').
text: http://www.w3.org/TR/2007/WD-WCAG20-20070517/#textdef
non-text content: http://www.w3.org/TR/2007/WD-WCAG20-20070517/#non-text-contentdef