[html] CSS: background-color only inside the margin

I have searched for an answer but couldn't find it anywhere. My question is reasonably simple: I have a background color of my body, then a large margin, and now I want a different background color inside the margin.

How do I do that with CSS?

That is not possible du to the Box Model. However you could use a workaround with css3's border-image, or border-color in general css.

However im unsure whether you may have a problem with resetting. Some browsers do set a margin to html as well. See Eric Meyers Reset CSS for more!

html{margin:0;padding:0;}

I needed something similar, and came up with using the :before (or :after) pseudoclasses:

#mydiv {
   background-color: #fbb;
   margin-top: 100px;
   position: relative;
}
#mydiv:before {
   content: "";
   background-color: #bfb;
   top: -100px;
   height: 100px;
   width: 100%;
   position: absolute;
}

JSFiddle

Instead of using a margin, could you use a border? You should do this with <div>, anyway.

Something like this?

http://jsfiddle.net/GBTHv/