Aside from explicitly noting that width/height may be negative, the spec is silent as to the sign of any particular attribute when that attribute is zero, for get

Presumably because of the DOMRect constructor lets you create a DOMRect with arbitrary x/y/width/height, you can have -0 for anything.  If so, it seems like it might be best to explicitly note that any field might be -0.

Additionally, because the language defining a Rect's origin introduces a sharp distinction for negative/non-negative width/height -- "When the rectangle has a non-negative width dimension, the rectangle’s horizontal origin is the left edge; otherwise, it is the right edge." and mutatis mutandis for height -- that language could use evaluation in light of -0, too.

I...somewhat think this all is distinct from the presence of -0, as described by users of DOMRect.  I also filed https://www.w3.org/Bugs/Public/show_bug.cgi?id=28918 for the getBoundingClientRect()/getClientRects() side of the -0 question.