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"When matching a selector against a document which is in quirks mode, class and ID selectors must be matched ASCII case-insensitively against the classes and ID of the elements in the document." * Please xref "quirks mode" (defined in the DOM spec) * It says to only apply to elements in the document, but should also apply to elements not in the document. This matters for querySelector. http://software.hixie.ch/utilities/js/live-dom-viewer/saved/3011