Replicating the norwegian flag using CSS
Quick implementation idea
Create a container div with two divs inside. Position those divs as a cross lust like in the Norwegian flag. After that give those divs a thick white border and the flag's red color would be simply the container div's background-color.
Code review

Sections are not an appropriate HTML element in this case. Use div's instead.
Do not style using html tags like body. Use CSS classes instead. Read about the css specificity
.red-square2, .red-square4{ this selector has both margin and margin-righ properties at once. margin: a b c d should be enough.
Tidy up your CSS. In one line You style .red-square3 and some lines below You style the same element. Why? Can't you use one CSS rule for that? :)

Improvement ideas

Move the CSS color to variables and place them in a separate file.