Document-centric XML document creation is a process of marking up textual content rather than typing text in a predefined structure. It turns out that, although the final document has to be valid with respect to the DTD/Schema used for the encoding, the "in-progress" document is almost never valid. At the same time, it is important to ensure that at each moment of time, the editor is working with an XML document that can be enriched with further markup to become valid. In this paper we explain the notion of potential validity of XML documents, which allows us to distinguish between XML documents that are invalid because the encoding is incomplete and XML documents that are invalid and no further encoding will make the document valid. We show that the set of potentially valid XML documents with respect to any DTD is context-free and we give a linear-time algorithm for checking potential validity for documents and document updates.