This question has been closed/held and marked off-topic: https://gis.stackexchange.com/questions/142117/what-are-dtd-schemas
Now, because I have a vague idea of what a DTD is, I can understand why. However the asker obviously doesn't know what a DTD is, hence the question. I suspect they came across it in a GML file (or other XML based GI format) and that's why they asked here.
Unfortunately for the user, the outcome of their question is that it is on hold and there's no feedback to the user as to why apart from it not being "on topic" and to ask on Stack Overflow.
Personally, were this to happen to my question when I was new here, I'd assume a bureaucratic and user-hostile community and stay away, but maybe I'm over-sensitive.
So, what's the best way to treat these questions?