In the post Feeds and well-formed XML Sean Lyndersay of the IE RSS team writes
Our years of experience in with HTML in Internet Explorer have taught us the
long-term pain that results from being too liberal with what you accept
from others. Hence, we’ve adopted the following overriding principle for IE
7 and RSS platform in Windows Vista:
We will only support feeds that are well-formed XML.
This principle allows us to build a more predictable feed parser. As a
platform, it's important that applications using the platform to consume
feeds can rely on the fact that the platform will always be providing
information in the way that the publisher intended (trying to guess what a
publisher meant to do when there is an error in a feed can be tricky, at best).
We also spoke to several people in the RSS and developer community at Gnomedex
and at PDC, and they wholeheartedly supported this.