I have an incomprehensible frame around the site?

Question

Web development

here is the picture how to fix it...

Answer 1

body { margin: 0}?

Answer 2

This can be caused by the collapse of the elements. Google "margin collapsing".
https://webref.ru/course/block-model/margin-collapse
Make all elements have a non-zero margin and you will see where the problem is.
For better visual convenience, make the elements of the site multi-colored.
After correcting the situation, do not forget to correct the design back.