Role of schema in XSLT?

I have a quesiton regarding what role a schema (DTD or XML Schema)
plays in using XSLT. Consider a DTD which defines an attribute as 
follows:

   <!ATTLIST ELEMENT ATTRIBUTE CDATA #FIXED "DEFAULT">

How does this attribute participate in the XSLT transformation?
Command line Saxon requires only the source XML document and XSL 
stylesheet:

   saxon source.xml style.xsl >output.html

But an attribute as defined above may not occur within the source
XML document. Rather, the attribute and its value are supplied by 
the DTD. Moreover, would an XML document which omitted the attribute 
be considered valid if everything else was in order?

Regads,

Dan


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