XML evangelists, such as myself, take great glee in pointing out that XML, unlike SGML, has no optional features; the result, we claim triumphantly, is that any XML processor in the world should be able to read any XML document in the world (well, modulo character encoding issues).
"Aha!" claim some ungrateful doubting Thomases; "XML distinguishes well-formedness and validity, and that's an option!"
Wrong. Anything that's well-formed is an XML document, and any XML processor has to be able to read any well-formed document. If a document wants to aspire to the higher karmic plane of validity, well good on it, but that's an extra, not an optional feature of XML.
Copyright © 1998, Tim Bray. All rights reserved.