Stylesheets have been used to convert the document type of SGML documents. With a stylesheet a document conforming to a source grammar can be transformed into a document conforming to a target grammar. This paper discusses the following problem: Given a stylesheet, a source and a target SGML grammar, is it decidable whether or not all documents conforming to the source grammar are transformed into documents conforming to the target grammar? Using context-free-extended context-free grammars we give a decision procedure for this problem.