Answer 1

Basically both work the same. There are some small differences about how they fit in width and size by default (as far as I've heard, but I have no proof or certainty of it).

In any case here is an important reference:

[1] "It is recommended that authors of HTML documents specify the canvas background for the BODY element rather than the HTML element"

[1] https://www.w3.org/TR/css-backgrounds-3/#special-backgrounds