OASIS Open Document Format for Office Applications (OpenDocument) TC

Some specific examples of how and why arbitrary proprietary extensions are 
evil.

Two common concerns with users is the need for privacy and security.  The 
issue of personally-identifying meta-data is increasingly in the news. 
Some products, like Microsoft Office, have a built-in operation that will 
remove such information from a Word document.  There are also third-party 
application that will strip such metadata from a document.

So, suppose you want to write such an operation for an ODF document.. What 
do you do?  Simple enough, look to meta.xml scrub extension elements under 

An interesting topic.

I will say that we are not talking about necessarily arbitrary nor
proprietary extensions (and the proposed requirement to document deviations
sounds like an interesting gate).  I am not otherwise going to address the
good-vs-evil argument.  

In existing language, there is the requirement that after the described
process of elimination occurs, there be a strictly-conforming document
underneath.  I think that is strong guidance to implementers about playing
safe and also wanting to maintain interoperability (if they so desire) with
conforming consumers that will not interpret the extension.  Also, adding
strictly-compliant documents should assuage those concerns that very
communities of practice and interoperability might have about the damage to
interoperability that foreign-elements and attributes might represent.

I can see a variety of useful ways to make use of such a thing with the
understanding of what the likely foreign-element treatment will be.  I can
also see how such provisions help with down-level use of up-level documents
and even with first-step ODF 1.2 implementations that, say, haven't figured
out how to support