I pulled up my rss feed in IE today, and got an error:
"Use of default namespace declaration attribute in DTD not supported."
Apparently, IE doesn't support the very valid code that specifies the document type in an xml feed.
So, while w3.org recommends specifying a doctype in every webpage, I had to pull my doctype specification in my RSS/XML feeds.
It should be working fine now. If not, drop me a line. I'll keep an eye on it over the next few days.
This is probably OK. The doctype I was using was XHTML, which XML should fit, but for example I can't guarantee that the code posted by my blog users will be strictly XHTML-only, which would break the doctype.
Lesson learned: even if you are writing valid, standardized, even recommended code, check first to make sure Microsoft decided to support it in IE. And periodically re-check your code to make sure Microsoft didn't decide to suddenly STOP supporting valid coding practices after the fact.