I ran into some issues while developing a site recently. Specifically, implementing an RSS feed of the clients news items. The XML validated perfectly fine and opened up in my testbed of various online and dedicated feed readers. However, the client insisted that something was broken. After much head-scratching and self-doubt I ascertained that the issue came down to the use of IE7 to view the news feed.
I never knew this before but IE7 will refuse to display any feed that uses a DTD. Apparently DTDs are insecure and every single one is untrusted. Yes, even those provided by the W3C.
I think that this says everything that you need to know about Internet Explorer’s attitude toward the foundation that develops free and open web standards.