Aaron, Dave and Brent have been discussing Postel's law and XML.
Some people have said that the issue with not well-formed feeds and aggregators is a kind of prisoner�s dilemma.
In other words, if all aggregator developers pledged to reject not well-formed feeds, then that would be incentive for people to fix their feeds.
It is a kind of Prisoner's Dilemma, but it is an iterated one. If the aggregators are relatively forgiving, users will trust them, but poorly formed feeds will end up somewhat mangled in different aggregators, leading to less trust in the feeds. Over time, the forgiving aggregators and clean feeds win out.
The power of forgiveness has been demonstrated experimentally.