a series of blocks at different pixel sizes, to test at which physical size (as measured on the device screen itself with a ruler) these are rendered on most common devices. measurements should be taken with default settings.
this should demonstrate how variable and volatile the relationship between CSS sizing and actual on-screen rendering is. see also pt versus px, which demonstrates that even "physical" units of length in CSS do not render consistently in terms of real-world physical size, since they are anchored on the CSS reference pixel.
note that this page uses <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no"> to force an "ideal" viewport.