Xml tab let's you validate some xml code and optionally also against an xml schema.
In the error list tab you can find the result of the validation.
If something turns red, then check out the list to see what's wrong.
// XML
<author xmlns='urn:bookstore-schema' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
<first-name>Eugenio</first-name>
<last-name>Favalli</last-name>
</author>
// XML schema
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="urn:bookstore-schema" elementFormDefault="qualified" targetNamespace="urn:bookstore-schema">
 <xsd:element name="author" type="authorName"/>
 <xsd:complexType name="authorName">
   <xsd:sequence>
     <xsd:element name="first-name" type="xsd:string" />
     <xsd:element name="last-name" type="xsd:string" />
   </xsd:sequence>
 </xsd:complexType>
</xsd:schema>