One difference between XML Schema 1.0 and other schema languages is that XML Schema 1.0 explicitly defines the information which a validating schema processor can and should (or must) provide to downstream applications: the post-schema-validation infoset (PSVI).

Some XML Schema processors (e.g. xsv and Xerces J) provide access to the PSVI in the form of an XML document. (It is, after all, structured information).

One thing (out of many) that you can do with this XML dump of the PSVI is to feed it through XSLT to generate -- well, something that looks like XML.

This may be the world's most expensive identity transformation. What's it good for?

If we just use a stylesheet to reflect the [validation attempted] and [validity] properties in the output, we can make it relatively simple to locate validity problems in the input.

The example here uses a visual display of the document to make the point; other applications of the principle could use sound or other characteristics in the output.