During the OOXML standardization proceedings, the ISO particpants felt that there was one particular sub-technology, Markup Compatibility and Extensibility (MCE), that was potentially of such usefulness by other standards, that it was brought out into its own part. It is now IS29500:2009 Part 3: you can download it in its ECMA form here, it only has about 15 pages of substantive text.
The particular issue that MCE address is this: what is an application supposed to do when it finds some markup it wasn't programmed to accept? This could be extension elements in some foreign namespace, but it could also be some elements from a known namespace: the case when a document was made against a newer version of the standard than the application.