"Invisible XML" is a project that is able to take any text document parseable by a context-free grammar and turn it into an XML document. Then it could be processed by any XML tools in a processing pipeline and even turn the result back into a non-XML document.
The key insight is that a parser for the grammar of the document can output an abstract syntax tree, and any AST can be represented by an XML document. From the announcement on
xml.com:
"Invisible XML,
as explained right here on XML.com, is a language for describing the implicit structure of data, and a set of technologies for making that structure explicit as XML markup. It allows you to write a declarative description of the format of some text and then leverage that format to represent the text as structured information."