Negative margins are in many cases equivalent to position:relative; with negative position, e.g. position:relative; top:-100px, as in Guffa's answer.

Negative margins get removed by Gmail and others. So, no negative margins.

You can't use negative margin in html email. To mimic this, there are 2 ways to do it, the nested tables way and the more complex rowspan way:

gmail and other mail services are ignoring the negative margin.

Negative values are mostly unsupported in html email. So is CSS position. For webmail at least, this is so that your email doesn't render outside of the desired window. Imagine Gmail with your CSS or email affecting the interface - they've limited the CSS you can use specifically to prevent this.