CSS: DIV-Box should always remain at the upper Browser Border

This is possible with pure CSS without any problems.

Just try the following CSS:

#immeroben{
  left: 0;
  top: 0;
  width: 100%;
  height: 50px;
  position: fixed;
  z-index: 999;
}

With the following HTML:

<div id="immeroben">INHALT</div>

With position:fixed it is possible to ensure that your element will be fixed and not scrolled with the page.

With left:0 and top:0 you are positioning the element at the upper left corner and using 100% and height:50px you are setting a width covering the whole page using a height of 50 pixels.

Last, you should select a high z-index value that must be higher than any other z-index of an element on your page ensuring that your DIV will always be at the top of all other elements. 
2015-09-30 at 13:39