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2d.imageData.object.round.html expects flooring for decimal values. AFAICS, this is not up to date any more. I conclude that there's the following chain: ctx.getImageData() returns an ImageData object whose data property is of type Uint8ClampedArray. According to: http://www.khronos.org/registry/typedarray/specs/latest/#7.1 Uint8ClampedArray uses WebIDL's _Clamping_ semantics, not EnforceRange semantics. Looking at http://www.w3.org/TR/WebIDL/#Clamp , we read: "See the rules for converting ECMAScript values to the various IDL integer types in section 4.2 for the specific requirements that the use of [Clamp] entails." which then redirects to 4.2.5.3 (the Clamp case, not the EnforceRange) case: http://www.w3.org/TR/WebIDL/#es-octet which mandates rounding, not flooring. Hence, the expectations in the patch should be changed to nearest values, not floored values.
Actually, correction: The test assertions actually don't expect flooring any more, they expect rounding, plus choosing the even integer when exactly in the middle (i.e. +0.5). But the description still needs to be updated and should contain newer spec references.