Properties of HTML style

I'm slightly confused about where I should be setting properties which
are global to an entire HTML file, and precisely which ones are
inherited by other elements.

For instance, if I wanted to have a global 6em left margin, what would
the effects be of setting this in either HTML or BODY? As I read the
spec, setting it on BODY would make every subsequent element inherit the
margin, thus paragraphs would have a margin of 12em (6 for the inherited
margin, 6 for the body's margin) - effectively the stylesheet becoming
(after inheritence):

BODY { margin-left: 6em; }
P { margin-left: 6em; }

It isn't clear from the spec what I should expect when setting the
margin on the HTML tag.

Thanks for any help,