The The usage of validation phases is a unique feature of ISO-Schematron and can be a very powerful tool e.g.
for establishing validation stages or to provide different validators for different "validation audiences".
If you ever need to pass an XPath as argument to the XSLT stylesheet you can pass in an etree.
XPath object (see XPath and XSLT with lxml: Stylesheet-parameters for background on this).
All you have to do is use a parser that has DTD validation enabled: option, which loads the DTD and weaves attribute default values into the document.
Again, no validation is performed unless explicitly requested.
Each error is displayed in its own Output window and includes a useful error message and a corresponding line number explaining the nature of the underlying XML parsing or validation error.