When do I have to style the html tag

When do I have to style the html tag

Can somebody give a general answer about when it's good to style the html
tag.
I noticed in one of my previous questions when I had an empty body element
and I styled the body trying to position an image at the bottom right. It
appeared at the bottom right but it was because the viewport was at the
top the image shown only about 30% at the top right.
So to solve this I added html{height:100%} and the image really was
position at the bottom of the browser window.