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xmlxml-declaration

OK to put comments before the XML declaration?

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 />
Solution

  • 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).