document

The document finds a node-set in an external document, or multiple external documents, and returns the resulting node-set.

document( URI [,node-set] )

URI: An absolute or relative URI of the document to be retrieved. The URI may also include a fragment identifier.
node-set (optional): An expression pointing to a node-set in the external document that should be returned.

A node-set.

If the URI contains a fragment identifier and that fragment can be identified in the external document, that fragment will be treated as the root in the node-set argument's expression. If the node-set argument is omitted, the entire fragment will be returned.
If the URI argument is a node-set, and the second argument is present, each node in the node-set will be evaluated as a separate URI, and the returned node-set will be as if the document function has been called multiple times (each time with the same second argument just as given in the function call) and the resulting node-sets had been concatenated into a single node-set.
Other specific conditions exist with specified behaviors. See the XSLT 1.0 documentation for details.
Since the URI is relative to the XSL document, document("") would return the root node of the current document.

This function is an XSLT-specific addition to XPath. It is not a part of the core XPath function library.

Supported.