The definition of xsl:source-document in the XSD does not allow @streamable (default: "no").

  <xs:element name="source-document" substitutionGroup="xsl:instruction">
    <xs:complexType>
      <xs:complexContent>
        <xs:extension base="xsl:sequence-constructor">
          <xs:attribute name="href" type="xsl:avt"/>
          <xs:attribute name="use-accumulators" type="xsl:accumulator-names"/>
          <xs:attribute name="type" type="xsl:EQName"/>
          <xs:attribute name="validation" type="xsl:validation-type"/>
          <xs:attribute name="_href" type="xs:string"/>
          <xs:attribute name="_use-accumulators" type="xs:string"/>
          <xs:attribute name="_type" type="xs:string"/>
          <xs:attribute name="_validation" type="xs:string"/>
          <xs:assert test="exists(@href | @_href)"/>
          <xs:assert test="not(exists(@type) and exists(@validation))">
            <xs:annotation>
              <xs:documentation>
                <p>The <code>type</code> and <code>validation</code> attributes are mutually exclusive
                  (if one is present, the other must be absent).</p>
              </xs:documentation>
            </xs:annotation>
          </xs:assert>
        </xs:extension>
      </xs:complexContent>
    </xs:complexType>
  </xs:element>