Feeds that reference a DTD are not supported by the RSS Platform. A DTD is used to help XML parsers with validation of the document. However, DTD validation is a potential source of security issues for XML parsers, and validation is not required for feeds to work correctly in aggregators.
This is by no means news from Microsoft possibly with good cause. The WS-I Basic Profile disallows DTDs in SOAP messages, and just like SOAP many see RSS as a serialisation spewed out by tools and not a document lovingly crafted by humans, to which I say “feh”. Still it’s likely to take many others by surprise.
Coincidentally I’m currently trying to work out how well supported DTDs are by databinding implementations, and thus far they seem OK-ish, but I do wonder for how much longer. I guess we are just observing a defacto XML 2.0 emerging, which is OK, but not before we have a standard replacement for entities, something like xmlchar or EDML might be good enough if only everyone could have agreed upon them six years ago. You know it’s much easier to chop things off than add new stuff in later.
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