how do i...

[Heinous]

No your wrong, for it to be valid css you don't.
http://jigsaw.w3.org/css-validator/

And it's 0px not 0.

Yes most browsers assume it's px, but not all.

Re-read what I said, now why did I say it?

[YouFail]

If you want it for left and right you can do it in one tag.

body {
margin: 0 0 0 0px;
}

It works.

[omgDAN!]

Re-read what I said, now why did I say it?

Because your not a coder who codes valid.
And all valid coding is cross-browser.

[Heinous]

Because your not a coder who codes valid.
And all valid coding is cross-browser.

Oh, right, I don't code validly.. I forgot. I mean, ask ANYONE who I've coded for, and at time of giving them the files, it was 100% valid CSS and XHTML 1.1, no warnings or errors.

And no, not all valid code is cross-browser. Get your facts right.