In looking at the streamability of the xsl-to-json() we found that it is difficult to see how to validate the streamed input without creating an in-memory copy. If the stylesheet did the xsl:stream instruction itself then it could invoke validation at this point, but if it accepts streamed input from its caller then it cannot do this.

Also, xsl:stream can only validate the entire input stream, not selected parts of it.

Apart from xsl:stream, the only way to invoke validation is

<xsl:variable name="x">
  <xsl:copy-of select="..." validation="strict"/>
</xsl:variable>

and this creates a grounded (in-memory) copy.

A possible solution might be a validate() function with the property that if the input is a streamed document, then the output is a streamed document, so one could do for example

xsl:apply-templates select="validate(.)"

to apply validation to a stream without grounding it.

Another solution might be a more general mechanism for multi-phase streamed processing, this might allow something like

<xsl:flow>
  <xsl:document>
    <xsl:copy-of select="." validation="strict"/>
  </xsl:document>
  <xsl:apply-templates select="."/>
</xsl:flow>

where each child instruction of xsl:flow takes the result of the previous instruction as its input (i.e. as the context item)