UCS PUA and charset tags

[Misha Wolf <misha.wolf@reuters.com>]

I recall seeing a mail (from Ned?) stating that any uses of the UCS 
Private Use Area (PUA) would require their own charset tags, but can't 
remember which list it was on.  I'm asking because MathML makes heavy 
use of the PUA and, presumably, uses the standard charset tags.

A follow-on question is:

If use of the UCS PUA requires additional charset tags when the text 
is encoded using one of the UCS-based CESs (eg UTF-16), what happens if 
the text is encoded using a more basic charset (such as US-ASCII), 
coupled with rules provided by a higher-level protocol, eg XML's 
numeric character references?

Misha


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