User-Agent: Opera/7.54 (X11; Linux i686; U) [en] Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.5) Gecko/20041103 Firefox/1.0RC2 If the contents of a popup are dynamically written, and the XHTML strict doctype tag and XML prolog are written with separate document.write commands, Mozilla/ Firefox/Gecko will use Quirks mode to render the popup contents even though it should use standards compliant mode. Checking the source of the popup correctly shows that it was all written correctly, but it still uses the wrong mode. Reproducible: Always Steps to Reproduce: 1. open a popup 2. use document.write to write its contents, using separate document.write commands for the XHTML strict doctype tag and the XML prolog 3. Mozilla/Firefox/Gecko will use the wrong rendering mode Actual Results: Quirks mode is used for rendering popup contents Expected Results: Standards compliant mode should have been used to render popup contents

Confirming. I'm not entirely certain when we sniff for a doctype in the parser, but I have a suspicion that we only check on the first nsParser::Parse(nsString ..., ...) so the two document.write()s throw us off, or something like that.

That's correct. We only look for the doctype in the first chunk of data the parser gets; the behavior noted in this bug can be reproduced with network packets too (simply by having a few kilobytes of whitespace between the XML decl and the doctype). We can't simply assume anything starting with an XML prolog is standards mode, though... if nothing else, some of it needs to be almost standards.

(In reply to comment #2) > the behavior noted in this bug can be reproduced with network > packets too (simply by having a few kilobytes of whitespace between the XML decl > and the doctype). Bug 261892 describes such a situation.