Calling insertBefore on Documents

Microsoft XML Core Services (MSXML) 5.0 for Microsoft Office - DOM Reference

If the current node is a document, calling insertBefore has the following results, depending on the value of the newChild parameter.

Node type passed in newChild	Result
`NODE_ATTRIBUTE`, `NODE_CDATA_SECTION`, `NODE_DOCUMENT`, `NODE_ENTITY`, `NODE_ENTITY_REFERENCE`, `NODE_NOTATION, NODE_TEXT`	Returns an error. These nodes are not valid as children of a document node.
`NODE_COMMENT`, `NODE_PROCESSING_INSTRUCTION`	Inserts `newChild` and returns `newChild`.
`NODE_ELEMENT`	Inserts `newChild` and returns `newChild`. By definition, an XML document (the Document node) can have only a single child. Therefore, an error is returned if the Document node already has a child.
`NODE_DOCUMENT_FRAGMENT`	Inserts the children of the document fragment (in `newChild`) and returns `newChild`. The insert operations are subject to the rules for child nodes and can fail if the document fragment children represent node types that cannot be inserted.
`NODE_DOCUMENT_TYPE`	Returns an error. The document type is read-only.