The W3C, which is responsible for the term well-formedness, defines it this way in the XML 1.0 recommendation (I'll take a look at each of these stipulations later):
A textual object is a well-formed XML document if:
Taken as a whole, it matches the production labeled document.
It meets all the well-formedness constraints given in this specification (that is, www.w3.org/TR/REC-xml).
Each of the parsed entities that is referenced directly or indirectly within the document is well-formed.
The W3C calls the individual specifications within a working draft or recommendation productions. In this case, to be well formed, a document must follow the “document” production, which means that the document itself must have ...