Answer 1

The CSS pixel is a rather complex unit, some people call it „magic unit“. The W3C CSS Reference gives us a pixel definition:

„The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm’s length. For a nominal arm’s length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm’s length, 1px thus corresponds to about 0.26 mm (1/96 inch).“
(reference: https://www.w3.org/TR/css-values-3/#reference-pixel)

So a CSS pixel is typically completely different from a physical hardware pixel. The pixel unit is defined to be small but still visible.