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To configure the XSLT options, open the Preferences dialog box (Options > Preferences) and go to XML > XSLT-XQuery > XSLT.

Oxygen XML Author offers the possibility of using an XSLT transformer implemented in Java (other than the XSLT transformers that come bundled with Oxygen XML Author). To use another XSLT transformer, specify the name of the transformer factory class. Oxygen XML Author sets this transformer factory class as the value of the Java property: javax.xml.transform.TransformerFactory.

The XSLT preferences page allows you to customize the following options:
JAXP XSLT Transformer - Value
Allows you to set the value of the TransformerFactory Java class.