If this is a duplicate of one of the existing DTD bugs, then please go ahead and
resolve it as such, though I'd appreciate it if you could help me with this
specific instance of the problem.

Here's the setup I inherited

xml/
    docs/
        myxml.xml
    dtds/
        myxml.dtd

And the system idendifier in myxml.xml is:
<!DOCTYPE MyXML SYSTEM "myxml.dtd">

myxml.xml is flagged as invalid in netbeans, because the DTD cannot be found.
So I tried (which would also be valid at runtime):
<!DOCTYPE MyXML SYSTEM "../dtds/myxml.dtd">

and got this error:

External entity not found:
"nbfs:DQCQBdevQB[elided]QBsource/xml/docs/../dtds/myxml.dtd". [3] 
XML checking finished.
Now, I'm not familiar enough with XML to know if relative paths are definately
allowed in SYSTEM identifiers, but its certainly true that the setup I have has
to be reasonably common. I tried a few varients for the SYSTEM identifier,
including the full path from the project root "directory" (ie,
/xml/docs/dtds/myxml.dtd) but nothing I tried worked other than copying
myxml.dtd into the same folder as the xml file...

There needs to be some way to specify where to search for DTDs, which would
preferably be linked from the error message that comes up when one can't be
found. If relative paths are allowed in SYSTEM identifiers, then you need to
support them. Absolute paths relative to the roots of the compiler's classpath
(which is effectively the same thing as the root of the project, usually) would
be useful too.