Are multiple XML declarations in a document well-formed XML?

Question

Is having two XML declarations in the same document well-formed XML?

<?xml version="1.0" encoding="UTF-8"?>
<a>
 <?xml version="1.0" encoding="UTF-8"?>
 <b>
  hello
 </b>
</a>

I believe it is not, however I can't find a source to back me up on this.

From Extensible Markup Language (XML) 1.0

Definition: XML documents SHOULD begin with an XML declaration which specifies the version of XML being used.

The pesky word "should" is there. It says ideally the document starts with an XML declaration. It says nothing about having another one within the document.

The document type declaration MUST appear before the first element in the document.

This is close, but it doesn't talk about the XML declaration itself, even though it should come before it.

Answer 1

Only one XML declaration is permitted in well-formed XML, and it must be at the top if anywhere.

Must be at the top

See the definition of document in the Well-Formed XML Documents section of the XML Recommendation:

[1]     document ::= prolog element Misc*

Then check prolog:

[22]    prolog   ::= XMLDecl? Misc* (doctypedecl Misc*)?

And then XMLDecl:

[23]    XMLDecl  ::= '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'

So, we see that the EBNF permits an XML declaration at the top of the document.

Only one

Processing instructions...

[16]    PI       ::= '<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'
[17]    PITarget ::= Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))

...in general may occur elsewhere, but a second XML declaration is precluded by virtue of the definition of PITarget and this statement:

The target names " XML ", " xml ", and so on are reserved for standardization in this or future versions of this specification.