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I was comparing http://dom.spec.whatwg.org/#dom-treewalker-previousnode to the corresponding Gecko code, and the Gecko code has one difference: Step 2 substep 2 has an additional subsubstep: 5. Set "sibling" to the previous sibling of "node". I believe the Gecko code is correct and the spec is wrong, since step 2 substep 2 says "While sibling is not null, run these subsubsteps:" but the subsubsteps do not change the value of "sibling".
Thanks! https://github.com/whatwg/dom/commit/c9330560a67d689e6ccb2a085a3acec433fe9878