margin in body(css problem)

Hi

I have problem which contains css in rails.

Here's the sample
CSS:
body{
  padding:0px;
  margin:0px;
}
#test{
      background-color:red;
}
HTML:
<div id="test">
  test
</div>

Okey it should go to the TOP of the page and make test text with red
background. But before the test div it makes line which height is
about 10px. Why rails does that, and how I can overide it?

by the way it's only firefox problem, with ie rails do it right but
not with firefox