This actualy depends on the way how you're writing a document to a file and has nothing to do with DOM itself. The following example uses popular Transformer-based approach:
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
Document document = factory.newDocumentBuilder().newDocument();
Element element = document.createElement("tag");
document.appendChild(element);
TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer();
transformer.setOutputProperty(OutputKeys.METHOD, "html");
DOMSource source = new DOMSource(document);
StreamResult result = new StreamResult(System.out);
transformer.transform(source, result);
It outputs <tag></tag> as you're expecting. Please note, that changing the output method has other side effects, like missing XML declaration.