The declaration is the element with <?xml version="..."?>.
https://www.w3.org/TR/xml11/#sec-prolog-dtd
It's required for XML 1.1. However, we'll always be using XML 1.0 because that's all libxml2 supports.
https://discourse.gnome.org/t/does-xmlnewdoc-support-xml-1-1/19329
See also https://github.com/ClusterLabs/pacemaker/pull/3323#discussion_r1457733427
If we do this, perhaps advertise UTF-8 encoding as well, something like
<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
The problem is that pcmk__xml2fd takes an xmlNode *, not an xmlDoc *. Printing an XML_DOCUMENT_NODE is what causes the version to be printed out. So we'd either need to change that function or make a new one, and then have xml_finish pass the document pointer instead of the root node.
This gets the right result (well, with an extra newline) but I don't like it:
diff --git a/lib/common/xml.c b/lib/common/xml.c
index 1e1b830a5..9fec40261 100644
--- a/lib/common/xml.c
+++ b/lib/common/xml.c
@@ -1678,7 +1678,7 @@ pcmk__xml2fd(int fd, xmlNode *cur)
xmlOutputBuffer *fd_out = xmlOutputBufferCreateFd(fd, NULL);
CRM_ASSERT(fd_out != NULL);
- xmlNodeDumpOutput(fd_out, cur->doc, cur, 0, pcmk__xml_fmt_pretty, NULL);
+ xmlNodeDumpOutput(fd_out, cur->doc, (xmlNodePtr) cur->doc, 0, pcmk__xml_fmt_pretty, NULL);
success = xmlOutputBufferWrite(fd_out, sizeof("\n") - 1, "\n") != -1;