How to add automatic addition of the squares on CSS?

There was a small problem in the CSS add inside the square the squares smaller and a different color. I decided this problem so
But the question was asked how to add the squares and it does not "wipe elbows"? Ie understand it is not to define width and height for the child:
Please tell me how to automate the process of adding subsidiaries of the squares?
April 7th 20 at 15:33
1 answer
April 7th 20 at 15:35
Well, strictly speaking, to solve the problem "in the CSS to add in the square the squares smaller and a different color" just one div element (no need to add a new decorative element on each square).

But still better to clarify I have asked the question, what does "erase the elbows" in his understanding.

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