I notice that the mobileread Wiki page you link to specifies a margin in the pt unit. Kinda funny given the talk here.
I don't include html in an epub stylesheet. For webpages I put this on the html tag. It's something the browser probably does anyway but it just makes sure.
Code:
html {
box-sizing: border-box;
}
As for what the difference is between html and body, it's one of those things that you need to have a lot of time to look into... and is not really worth worrying about in the end.