XML attributes

To Whom It May Concern:
Hi,
I have a question about the " Xml Attributes"
How could I check the null or minOccurs or maxlength with XML or XML Schema 
or DTD? When I change the value to null or the value exceeds the maxlength 
constraint, parser does not raise any error, why? (Parser: 
MSXML)<?xml:namespace prefix = o ns = 
"urn:schemas-microsoft-com:office:office" />

For example:
…
<s:AttributeType name="CharacterName" rs:number="2" rs:nullable="false" >
     <s:datatype dt:type="string" dt:maxLength="2"/>
</s:AttributeType>
…
         <z:row ID="1" CharacterName="John Smith" />
     </rs:data>
</xml>
Thanks to everyone who has helps with this draft!
Regards,
Shahrzad