The first line of an XML document must not be blank, since the document will fail the well-formedness test, and cannot be a valid XML document. As described in the standard, an XML document should begin with an XML declaration which specifies the version of XML being used. (http://www.w3.org/TR/2008/REC-xml-20081126/#sec-prolog-dtd )
However, a document not declaring the XML version on the first line can still pass the well-formedness test, but it cannot be a valid document. If the first line is blank, it cannot be well-formed, nor valid.
For example, the following is a well-formed XML document, but not valid:
<?xml version="1.0"?>
<foo>This is the content of the foo element</foo>
This can be tested in an XML validator; for example, the one found at http://www.validome.org/xml/validate/:
As visible from above, the document is not valid because nowhere is the <foo> element declared. However, if we only test for well-formedness:
So summarize, the first line of an XML file should be used to specify the version of XML used; but it can also be any element, as long as it is not blank. However, a document without the declaration of the version of XML used cannot be valid – it can be well-formed, assuming XML syntax is respected throughout the file.
