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When validating the above URL, the error comes up as Line 1, column 62: no system id specified . But there is no explanation under that. The owner of that page has a DOCTYPE, but the DOCTYPE is missing the URL of the DTD.
This was not, per se, a bug, since many error messages, especially rare ones, do not have any additional explanation. We have a specific mechanism for users to contribute to the pool of explanations. I have added an explanation for the warning you mention, see: http://dev.w3.org/cvsweb/validator/htdocs/docs/errors.html.diff?r1=1.34&r2=1.35
This error/warning should show up for HTML, too, not just XML-based. I constantly run into people using Transitional HTML doctype declarations without a system identifier and they don't understand why their layouts are strangely rendered in foo browser (because quirks mode has been triggered).
(In reply to comment #2) > This error/warning should show up for HTML, too, not just XML-based. I > constantly run into people using Transitional HTML doctype declarations without > a system identifier and they don't understand why their layouts are strangely > rendered in foo browser (because quirks mode has been triggered). Validation has nothing to do with browsers' proprietary behaviors. Please send a bug report to browser vendors if you feel valid markup trigger "quirk"y behavior in browsers