Question

Is it OK to put comments before the XML declaration in an XML file?

<!--
Is this bad to do?
-->
<?xml version="1.0" encoding="utf-8"?>
<someElement />

Answer 1

No, it’s not OK.

Appendix F of the XML spec says:

Because each XML entity not
accompanied by external encoding
information and not in UTF-8 or UTF-16
encoding must begin with an XML
encoding declaration, in which the
first characters must be ‘< ?xml’, any
conforming processor can detect, after
two to four octets of input, which of
the following cases apply.

Ah, but, section F is non-normative, you say.

Well, section 2.1 gives the production for a well-formed XML document, thus:

[1]     document       ::=       prolog element Misc*

…and in section 2.8 we get the production for “prolog”:

[22]    prolog     ::=       XMLDecl? Misc* (doctypedecl Misc*)?
[23]    XMLDecl    ::=      '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'

So, you can omit the < ?xml declaration, but you can’t prefix it with anything.

(Incidentally, “Misc” is the category that comments fall into).