Answer 1

I wrote
'background: fixed;'
a line before 'background-image' and it worked.

I guess that's because you didn't allocate background.

Answer 2

This is because you are using position: absolute on the body; and he falls out of the general flow.
html does not see the body and is folded because it is not sized, (the default html size is set depending on its content).
You need to set the html size like this:
html {
height: 100vh;
}