--> All tags must be balanced
--> All attribute values must be  in quotes
--> Any EMPTY element tags must end with /> or make them 
non-empty by adding a end tag
--> there must not be any isolated mark up characters in ur 
text data
--> elements must nest inside each other properly
--> well-formed files with no dtd may use attributes on any 
element,but the attributes must all be of type cdata by 
default.