http://www.perlmonks.org?node_id=585903


in reply to Re^2: Config Files Redux
in thread Config Files Redux

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>
This allows
<box color=red />
but not
<box color=large />
The default is blue in this example.