XML is extensible as the new elements attributes etc can be easily added to the existing XML document without affecting the original XML document's structure.
Thus an already existing application will not break/crash due to newly added information.
The very purpose of XML is extension of the document. XML specfies few and simple rules like . The document should be a wellformed xml. . Must have a root element . An element must have end tag . Few other simple rules
Other than that XML doesnt levy heavy restriction like . The document must contain 'n' tags starting with xyz tag. (As html does first tag should be html header body etc) . No restriction on formatting of data (color font etc.)
Thus xml is easily extensible and the proof is WML mathXML ebML cXML
