The process of creation of document-centric XML documents often starts with a prepared textual content, into which the editor introduces markup. In such situations, intermediate XML is almost never valid with respect to the DTD/Schema used for the encoding. 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 enriched with further markup to become valid. In this paper we introduce the notion of potential validity of XML documents, which allows us to distinguish between XML documents that are invalid because the encoding is simply incomplete and XML documents that are invalid because some of the DTD rules guiding the structure of the encoding were violated during the markup process. We give a linear-time algorithm for checking potential validity for documents.