Screen, PDF, printer - strictly specified dimensions in centimeters. How to achieve absolute 100% dimensional accuracy?

Question

Books

Everyone, GOOD!
There are different blocks on a web page and I need to present them to users (output on a monitor, in an electronic document, on paper through printing on a printer) in a strictly specified size - in centimeters (millimeters, etc.).
That is, to be sure that by attaching a ruler to ANY IMAGE: a monitor or a printed document, the block sizes will remain the same and unchanged (relative to those originally set).
Is this really possible to achieve and how?
In advance, Thanks everyone!

Answer 1

Perhaps you need to specify the dimensions in millimeters

<div style="width:50mm;height:20mm;background:#000"></div>

This size is calculated by the browser taking into account DPI
. But we must take into account that the size is the inner part and the border is the outer one.

Answer 2

The easiest way is to calculate the conversion factor by simply printing (or displaying) a test ruler and measuring it with a reference ruler. The ratio between the test and reference (it may be different along the X and Y axes) is then used in the design of the web page. But...

...present them to users

Here an addition immediately suggests itself - to different users. with their huge zoo of different monitors and printers, each has its own conversion factor. You can't do it personally, so it's up to each of them to do it themselves. Well, you can hang on the site the appropriate instructions.