A number of CSS specifications reference the concept of "device pixels" or "hardware pixels". See, for example, 6.2. Absolute Lengths: the cm, mm, Q, in, pt, pc, px units in css-values-4, or 5.1. Display Resolution: the resolution feature in mediaqueries-4 (which uses "hardware pixels" and "device pixels" interchangeably).

As far as I can tell, they do so without defining the term. I think a brief definition would be appropriate.

However, there are other concepts, particularly related to rasterization, such as the proposal in #3720 or the discussion in #7284 that want to discuss details of rasterization. Some of these discussions contain the assumption that rasterization happens at a scale of 1 pixel in the rasterization being equal to 1 device pixel. This is usually the case, but it isn't always. For example, in the presence of animated transforms the rendering is often rasterized once and then that rasterization is animated across a range of scales before rerasterization occurs. Or for transforms that are not a translation, device pixels and raster pixels might have different axes and different scales.

These two discussions that I mention seem like they would benefit from a definition of "raster pixels" that could be used when discussing issues of rounding of sizes or snapping to edges. (Though I admit I'm not entirely sure how such a definition should handle the case of non-translation transforms.)