Asir S Vedamuthu has announced a W3C XML Schema for XSLT 1.0 that can validate the structure of XSLT stylesheets and can be used in conjunction with schemas of target vocabularies to pre-validate the generated documents.

The schema, using advanced object-oriented features of W3C XML Schema such as groups of elements and attributes and substitution groups, captures all the structural constraints of the XSLT 1.0 recommendation; except a point raised as a W3C XML Schema issue by James Clark, that has been solved by the latest PR (16 March 2001):

The wildcard facility in XML Schemas doesn't seem to handle the case of xsl:stylesheet, which allows specific elements from xsl namespace plus any element whose namespace URI is both not XSL and not absent.

It defines its own version of the QName simple type to match the definition from the XSLT specification, after a workaround for another comment from Clark:

It occurred to me that an XSLT-like QName datatype which doesn't apply the default namespace to unprefixed values (ie treats them like attribute names rather than element names) can be simulated in XML Schemas by a union of 1/ NCName, and 2/ a restriction of QName using the pattern facet to require a colon.

Literals in the stylesheets are allowed though a group of elements that can be redefined to perform some pre-validation on target vocabularies.

Although the schema; authored by Asir S Vedamuthu (webMethods) acknowledges the contribution of James Clark and is described in its comments as "a product of the W3C XML Schema WG Task Force for authoring schema for W3C Recommendations as an illustration of the XML Schema Language", it has been announced as the "webMethods' XML Schema for XSLT 1.0" and is available on the webMethods web site.