Describing / Declaring Properties

<?xml version="1.0"?>

     <!DOCTYPE rdf:RDF [<!ENTITY xsd
    "http://www.w3.org/2001/XMLSchema#">]>

     <rdf:RDF

     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"

     xml:base="http://example.org/schemas/vehicles">

    

     <rdf:Property rdf:ID="color"/>

    

     <rdf:Property rdf:ID="make"/>

    

     <rdf:Property rdf:ID="model"/>

    </rdf:RDF>

Note: can anyone identify a problem here?