The margin property can escape the parent node and move it alongside
its child. This happens with singlePage div and the body, resulting in
scrollbars appearing as the body has the size of the viewport but
does not align with it. This phenomenon can be always observed
in the vertical axis and it can also be observed in the horizontal axis
when the viewport it’s not wide enough (mostly in mobile).
Using paddings prevents this “extra space” from scraping and displacing
the body.
Also, the value 100vw does not take into account the space taken by the
vertical scrollbar, thus making the body wider than the actual viewport,
producing a horizontal scrollbar.
Having the CSS and JS in the html template produces pages larger
than necessary, as each page need to contain all the js/css.
Separating them in appropriate files allow the browser to just download
them once and use them for all the pages. This is even more effective
with an aggressive cache policy for the js and css, something that can
be done without fear thanks to the implemented cache-busting.
Also, having then in separate files allows us to use Hugo pipelines
for minimizing the code.
Claudio Yanes