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"5 Extensions to HTMLLinkElement Interface" says it is null when "The link does not represent an import" but it isn't clear what "represent" means. specifically it isn't clear about 404 case. "6 Loading Imports" section isn't clear either. It doesn't mention what should happen when UA cannot fetch the resource.
https://dvcs.w3.org/hg/webcomponents/rev/1d58eae952d8