The current CSS Color 4 spec doesn't define specific values for when a Lab/OKLab color should be considered achromatic, stating:

For extremely small values of a and b (near-zero Chroma), although the visual color does not change from being on the neutral axis, small changes to the values can result in the reported hue angle swinging about wildly and being essentially random. In CSS, this means the hue is powerless, and treated as missing when converted into LCH or OKLCh;

(https://drafts.csswg.org/css-color-4/#lab-to-lch)

The lack of specificity here (and therefore lack of testability) has lead to incompatibility between browsers (see https://bugs.webkit.org/show_bug.cgi?id=287637 for an example).

Is there some reason we cannot define what this "extremely small values of a and b (near-zero Chroma)" is? If defining specific constants is not ideal, perhaps some bounds?