I prefer XML with a DTD because it can help prevent size=blue or color=large type mistakes...

You've said this multiple times, but you've never explained how.

The how is the same way you would do it for any other type of file. Someone writes a comprehensive set of rules by which to validate the configuration file. The file format doesn't matter. The format of the rules doesn't matter. Something reads and processes the configuration file and the rules and applies the rules to the processed file.

You have to do this anyway to parse the configuration file in your application.

Now you may have better tools to do this with an XML application than with any other format, but then your real argument for using XML is better tools, not validation.

And tools like this exist for formats like YAML; for example you can use kwalify to produce a DTD-like schema for YAML documents.

If the DTD is a tool, then yes, the issue is better tools. The way to do it in a DTD is:

<!ATLIST box color (red|blue|green) blue>
[download]

This allows

<box color=red />
[download]

but not

<box color=large />
[download]

The default is blue in this example.

Pity that's not XML, though.

Worse, it means that everyone who edits the configuration file either forgoes validation or has to use an XML-aware editor. There goes scriptability and so many of the lovely Unix administration tools we've grown to love over the past few decades.