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This section: http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#storage-mutex does not reflect what any major UA implements or plans to implement in the near term (please correct me if I'm mistaken). It's marked as optional, so the only purpose this text serves is to confuse readers of the spec who think that it might have some bearing on behavior. The text should either be removed or replaced with something that reflects reality.
I'll add some much clearer text explaining that browsers have opted not to implement this for now but encouraging browsers to do so nonetheless.
Checked in as WHATWG revision r8503. Check-in comment: Add a note pointing out that browsers haven't implemented the storage mutex. http://html5.org/tools/web-apps-tracker?from=8502&to=8503