[css-fonts-4] [varfont] font-optical-size's relationship with zooming should be defined

[litherum]

Because browsers will likely use text's physical font size in addition with the device's dpi to determine font-optical-size, the affect of those things changing should be specified.

This is important because font-optical-size affects layout, but some zooming operations shouldn't affect layout.

[litherum]

Rather than enumerating all the different kinds of zooming operations UAs may perform, instead, a better idea would be to partition these operations into "layout-affecting" and "non-layout-affecting" operations. Then, the spec can simply say that UAs performing "layout-affecting" zoom operations may re-select a value for the optical-size variation, but may not re-select a value for "non-layout-affecting" operations.

[litherum]

d9ea1bd