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As a workaround for a different bug ("characters larger than nnn not supported"), the validator supresses error messages related to errors in DTDs (actually it supresses messages from any other source than the document beeing checked). This leads to the fun situation where the Validator reports a page as invalid but doesn't give any actual error messages. The correct fix is to deal with the original problem and begin reporting all errors again.
Marking 0.7 to put back on the radar.
The problem goes deeper than this. We can't report DTD errors without a change to output formats, but you knew that. But it's also a fix for *standard* DTDs that emit warnings, like W3C HTML 4.0. Reporting DTD errors starts to look a little like the difference between Page Valet and Code Valet. Different tools. So let this be a post-m12n goal.
Taking up Nick (and Björn's) suggestion and retargetting this for some point in the future. Fixing this will open up a can of worms I'm not sure we can close until well after M12N.
In validator 0.8.0, errors in the DTD are reported, although all errors are reported as if pertaining to document validated. Changing bug summary from: Errors in DTDs are Not Reported To: Errors in DTDs are not distinguished from errors in document
I see this also, with <!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.1//EN" "http://www.openmobilealliance.org/tech/DTD/xhtml-mobile11.dtd">