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The "should not" requirement about obsolete doctypes should probably be in the syntax section, written in the same way as the normal doctype and legacy-compat doctype, instead of being listed in the obsolete features section.
Why?
Because it seems reasonable to assume that the syntax section covers all allowed syntax. Readers are likely to come to the conclusion that those doctypes are invalid.
would that be a bad thing?
Yes. The spec should be clear about what the rules are. Otherwise people might give up trying to deduce rules from the spec (not just about syntax but in general).
EDITOR'S RESPONSE: This is an Editor's Response to your comment. If you are satisfied with this response, please change the state of this bug to CLOSED. If you have additional information and would like the editor to reconsider, please reopen this bug. If you would like to escalate the issue to the full HTML Working Group, please add the TrackerRequest keyword to this bug, and suggest title and text for the tracker issue; or you may create a tracker issue yourself, if you are able to do so. For more details, see this document: http://dev.w3.org/html5/decision-policy/decision-policy.html Status: Accepted Change Description: see diff given below Rationale: Concurred with reporter's comments.
Checked in as WHATWG revision r4808. Check-in comment: Define the obsolete permitted DOCTYPEs in the syntax section. http://html5.org/tools/web-apps-tracker?from=4807&to=4808