Various binary formats have been proposed as compact representations for XML (Extensible Markup Language).
There are several competing formats, but none has yet emerged as a de facto standard, although the World Wide Web Consortium adopted EXI as a Recommendation on 10 March 2011.
The major challenge for binary XML is to create a single, widely adopted standard.
The Open Geospatial Consortium provides a Binary XML Encoding Specification (currently a Best Practice Paper) optimized for geo-related data (GML) and also a benchmark to compare performance of Fast InfoSet, EXI, BXML and deflate to encode/decode AIXM.
There are also hybrid approaches (e.g., VTD-XML) that attach a small index file to an XML document to eliminate the overhead of parsing.