Unicorn: " XML declarations are problematic"

Hello.

When examining my website [1] I get this error:
³This document has not passed the test: W3C XHTML 1.0 Appendix C checker:
XML declarations are problematic. See HTML Compatibility Guideline 1:
Processing Instructions and the XML Declaration².

However, I do use ISO-8859-1 as the character set, so I believe I have to
include the XML declaration, as its absence makes UTF-8 or UTF-16 the actual
character set, an inconvenience for a non-English language.
I think Unicorn should only warn for a possible ³problematic² issue, but not
to mark its inclusion as an absolute error, provided:

1.- The Standard considers that this declaration SHOULD be included ­so it
could be supressed only when serious reasons concur.
2.- Omitting it when character sets other than UTF-8 or 16 are used may
cause unexpected problems, so I think it¹s convenient for ISO-8859-1

Ricardo de la Vega

[1] http://www.hipocampo.org/