These characters are part of the HTML+ spec, as described in:
ftp://15.254.100.100/pub/draft-raggett-www-html-00.ps
ftp://15.254.100.100/pub/htmlplus.dtd.txt
The Postscript write-up is some what dated, see the DTD for an upto date
description of HTML+. A new write-up is in preparation, and any feedback
is welcome.
Dave Raggett