W3C XML Schema has overtaken DTDs in mindshare
DTDs came with XML 1.0
A slimmed-down version of SGML DTDs, they were designed for the document applications XML's creators originally foresaw.
DTDs didn't support namespaces
Since Namespaces in XML appeared after XML 1.0, DTD's had no way of understanding namespaces. There have been some ingenious hacks to create DTDs which could cope with namespaces, but they're ugly.
Limited datatyping ability
To a DTD, everything is a string, or a string with very simple constraints. The DTD data types don't match the expectations of data-oriented developers very well.
Didn't feel like programming
DTDs define relatively simple structures using a simple set of constraints. It didn't feel like a powerful and flexible system for integrating data with processes.
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