Consider the schema: 

      <xsd:schema
        xmlns="http://schematests.com"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        targetNamespace="http://tests.com">

        <xsd:element name="root">
            <xsd:complexType>
                <xsd:sequence>
                    <xsd:element name="name" maxOccurs="unbounded"/>
                </xsd:sequence>
            </xsd:complexType>
            <xsd:key name="nameKey">
                <xsd:selector xpath="./name"/>
                <xsd:field xpath="."/>
            </xsd:key>

        </xsd:element>

      </xsd:schema>
Since no type is declared for the local "name" element, by the properties 
tableaus in [1], it must have the ur-type. 

Consider the instance document 


      <my:root
        xmlns:my="http://tests.com"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://tests.com idAnytype.xsd">
        <name>Jack Daniels<name>

        <name>Johnny Walker<name>

        <name>Sam Adams<name>
      <my:root>
From the 3rd point of [2]: 

"3 For each node in the target node set all of the {fields}, with that node as 
the context node, evaluate to either an empty node-set or a node-set with 
exactly one member, which must have a simple type. " 
[3] tells us that the ur-type can behave as a simpleType "according to 
context": 

"[Definition:] A distinguished ur-type definition is present in each XML 
Schema, serving as the root of the type definition hierarchy for that schema. 
The ur-type definition, whose name is anyType, has the unique characteristic 
that it can function as a complex or a simple type definition, according to 
context." 
This raises two questions: 

Is it valid for a <field> to match an element with the ur-type definition under 
any circumstances? 
If such a match may sometimes be valid (presumably when the element only 
contains textual content): 
If the element contains text, in which value space should the schema-normalized 
value of the field be considered to lie? This will be significant in the case 
of keyref matches, especially in light of the recent discussions concerning the 
incomparability of values from disjoint value spaces. 
I presume that an error should be raised if the instance of the ur-typed 
element actually contains element content? Or should the <field> match simply 
fail? 
[1]: http://www.w3.org/TR/xmlschema-1/#declare-element 

[2]: http://www.w3.org/TR/xmlschema-1/#section-Identity-constraint-Definition-
Validation-Rules 

[3]: http://www.w3.org/TR/xmlschema-1/#key-urType 

See:
http://lists.w3.org/Archives/Public/www-xml-schema-comments/2002JulSep/0085.html

Henry's response:
http://lists.w3.org/Archives/Public/www-xml-schema-comments/2002JulSep/0089.html