although w3c xml schema's ability to add default attributes to elements is an advantage, it is a disadvantage in some ways as well. it means that an xml file may not be usable in the absence of its schema, even if the document would validate against that schema. in effect, all users of such an xml document must also implement the w3c xml schema specification, thus ruling out minimalist or older xml parsers. it can also slow down the processing of the document, as the processor must potentially download and process a second xml file (the schema); however, a schema would normally then be cached, so the cost comes only on the first use.