As per mdn/content#28157, margin-box is no longer a possible value for mask-origin and mask-clip, but the formal syntax sections here and here still contain them.

The syntax for them can be found here and here, in both cases all they do is set <geometry-box> which contains <shape-box> and therefore margin-box as a property. Confusingly the W3 spec says they are still use <geometry-box> (here and here), which does still contain margin-box, yet also says margin-box is no longer a possible value for mask-origin and mask-clip (here), so I'm not sure what the best way to go about removing margin-box as a property would be.

<geometry-box> is defined here and <shape-box> here. I don't think margin-box can be removed as a property of <shape-box> as it's still a valid property for that, and other syntaxes that are supposed to support margin-box do use <shape-box> . One option might be to create another <geometry-box> and <shape-box> specifically for mask-origin and mask-clip, which don't contain <margin-box>. Another might be to manually list all of the possible values of mask-origin and mask-clip instead of using <geometry-box> to do this, but that wouldn't then match the W3 syntax which does use <geometry-box>.

Is there any way of removing/altering the properties created by using an object in syntaxes.json, ie removing margin-box while still using the existing <geometry-box> and <shape-box> objects? If so that seems like it'd be the best way of approaching this, but I don't know if that'd be possible or how it'd be done.