How about:
... A minimally conforming HTML user agent must support the SGML
declaration in section SGML Declaration for HTML, which specifies ISO
Latin 1 (@@full name) as the document character set. In addition, it
may support other SGML declarations which vary in accordance with the
following guidelines:
1) It is allowable for an HTML user agent to support an SGML
declaration with a different syntax reference character set,
but it should be noted that documents marked up in accordance
with such SGML declarations may, or may not be, processable by
other user agents.
2) It is allowable for an HTML user agent to support an SGML
declaration with a different document character set, though it
should be noted that in the future ISO 10646 will be adopted as
the document character set, thereby subsuming all SGML
declarations with a document character repertoire completely
ontained within ISO 10646.
3) The values found within the QUANTITY section represent minimum
values. Values above those defined are allowed, but it should be
noted that this behaviour cannot be counted upon by authors
of HTML documents.