How to make a line between grid blocks?

Can't figure out how to make these lines, please help
April 19th 20 at 12:36
2 answers
April 19th 20 at 12:38
This option with flex and offset a couple of pixels:

Thank you! - fae.Terr commented on April 19th 20 at 12:41
April 19th 20 at 12:40
Sad: the pseudo-elements to draw borders or make the pseudo-element white quadratic in the upper right corner of each block, and borders as usual.
Perverted version: how to use border-image.

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