You have duplicate DOCTYPE declarations. If you want to reference an external DTD:
test.xml
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE email SYSTEM "test.dtd">
<email>
<von>[email protected]</von>
<zu>[email protected]</zu>
<titel>Hello</titel>
<text>Dear John....;-).</text>
<prior type="schnell"/>
</email>
test.dtd
<!ELEMENT email (von,zu,titel,text,prior)>
<!ELEMENT von (#PCDATA)>
<!ELEMENT zu (#PCDATA)>
<!ELEMENT titel (#PCDATA)>
<!ELEMENT text (#PCDATA)>
<!ELEMENT prior EMPTY>
<!ATTLIST prior type CDATA #REQUIRED >
If you want your DTD as part of the XML file (internal subset):
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE email [
<!ELEMENT email (von,zu,titel,text,prior)>
<!ELEMENT von (#PCDATA)>
<!ELEMENT zu (#PCDATA)>
<!ELEMENT titel (#PCDATA)>
<!ELEMENT text (#PCDATA)>
<!ELEMENT prior EMPTY>
<!ATTLIST prior type CDATA #REQUIRED >
]>
<email>
<von>[email protected]</von>
<zu>[email protected]</zu>
<titel>Hello</titel>
<text>Dear John....;-).</text>
<prior type="schnell"/>
</email>
NOTE: You're also missing an ELEMENT declaration for your prior
element.
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