css - why use negative margins?

Question

Just looking at some CSS here, and I noticed:

.head{position:relative;overflow:hidden;margin:-30px 0 0 -25px;width:820px;padding:20px 25px 0 25px;background:url(/images/bkg.gif) 0 0 no-repeat;}

Why would you put -30 and -25px margins?

Answer 1

I started typing an answer, and then found a much better one here (Wayback Machine backup). Some salient points:

Negative margins:

• are valid CSS
• don't break page flow
• have high levels of cross-browser compatibility (although if they break your link or floated image, then try adding position: relative; that should fix it)

Their effect on unfloated elements:

• applying them to the top or left of an element "pulls" that element in the appropriate direction(s)
• HOWEVER, applying them to the bottom or right of an element "pulls" immediately subsequent elements into them, making them overlap

Their effect on floated elements:

• this is more complex and I can't summarise it better than the article. Have a play around in Firebug to get a feel for them.

There are some brilliant examples of negative margin use in that article (especially the 3-column layout! Magic. I've used a similar technique for page layout before.) The most common use for them I've found is just to move an element a small amount to correct its position, and to make one element overlap another for visual effect.