Attributes can be declared in the 'complexType' element via the 'attribute' element.

Example:

<xs:attribute name="href" type="xs:anyURI"/>

The 'use' attribute specifies optionality and can have values 'optional', 'required', 'prohibited'.

Don't worry about the value 'prohibited' for now...

In context:

<xs:element name="link">
<xs:complexType>
<xs:attribute name="href" type="xs:anyURI"/>
</xs:complexType>
</xs:element>