All text, names, values, syntax, etc. in an XML document are Unicode Characters.

Unicode assigns each character a "code point"--a number and arranges these code points into code blocks.

Fonts map glyphs to code points (hopefully correctly).

The familiar ASCII is the first code-block (e.g. decimal 65 is the letter 'A')

Unicode attempts to represent all languages with code blocks and code points.

Code points also have properties like: 'whitespace character' or 'name character'.

The Greek Code Page: Ux0370

Both Microsoft and Apple do not ship complete Unicode fonts even though their operating systems fully support Unicode. Complain!!!

Check out the STIX page or Unicode consortium's page more info on complete fonts.