Sam Ruby posted more today about XML error parsing tolerance in his Validate on subscription? post (which notably links to Brent Simmons' Compromise on Atom and XML). I think "negotiation" is an interesting concept to add to this discussion.

When an XML document (e.g., an RSS feed) fails to match an expected schema, possibly by not being well-formed, etc., the document reader can negotiate within its own context what parser (or parser features) it needs in order to successfully extract what it is looking for. The reader can remember what parser it uses for documents and can (try to) skip the negotiation step on future reads.

Another context of negotiation that I am exploring with the iCite net is one where the reader asks the feed producer for a certain format. For example, the reader can ask for well-formed XML or not.

In particular, I see this as being useful for when more or less markup is needed. With RSS as an example schema, I can imagine instances where having a full RDF (RSS 1.0) version would be useful. At other times (like a mobile phone feed reader perhaps), a minimalistic format like RSS 3.0 or ESF might be more approrpirate.

I guess the assumption with RSS is that it is harder to "ask" an RSS server to deal with the format of its RSS than it is to deal with this in the RSS reader. I guess, in some ways, fat clients live on!

With the iCite net, the model is more peer-to-peer, with the general case being that a reader is a server and vice versa. Maybe RSS readers will evolve towards this eventually as well—instead of subscribing to a feed from its original server, you might subscribe to it from your friend whose reader reformats and republishes source feeds as well-formed XML or RDF or text or whatever you need.

(And, I guess if iCites effectively work as RSS readers and republishers, then there will be at least one exaple of an RSS reader that tries to evolve this way ;-)