Declarative specification of XML document fixup

The historical and social complications of the development of the HTML family of languages defy easy analysis. In the recent discussion of the future of the family, one question has stood out: should ‘the next HTML’ have a schema or indeed any form of formal definition? One major constituency has vocally rejected the use of any form of schema, maintaining that the current behavior of deployed HTML browsers cannot usefully be described in any declarative notation. But a declarative approach, based on the Tag Soup work of John Cowan, proves capable of specifying the repair of ill-formed HTML and XHTML in a way that approximates the behavior of existing HTML browsers. A prototype implementation named PYXup demonstrates the capability; it operates on the PYX output produced by the Tag Soup scanner and fixes up well-formedness errors and some structural problems commonly found in HTML in the wild based on an easily understood declarative specification.