filter-src

<xs:attribute name="filter-src" use="optional">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:pattern value="url\([ ]*'[^']+'[ ]*\)"/>
      <xs:pattern value="url\([ ]*"[^"]+"[ ]*\)"/>
      <xs:pattern value="url\([ ]*[^"']+[ ]*\)"/>
      <xs:pattern value="buffer\([ ]*.+[ ]*\)"/>
    </xs:restriction>
  </xs:simpleType>
</xs:attribute>