How to remove whitespace that appears after relative positioning an element with CSS

You can simply solve this by applying a negative margin that equals the width or height of the element.

For an element of 100px height that is positioned to the top you will apply margin-bottom:-100px;

For an element of 100px height that is positioned to the bottom you will apply margin-top:-100px;

For an element of 100px width that is positioned to the left you will apply margin-right:-100px;

For an element of 100px width that is positioned to the right you will apply margin-left:-100px;

cut & paste css snippets:

.top 
    {
    postion:relative;
    top:-100px;
    height:25px;
    margin-bottom:-25px;
    }
.bottom
    {
    postion:relative;
    top:100px;
    height:25px;
    margin-top:-25px;
    }
.left
    {
    postion:relative;
    left:-100px;
    width:25px;
    margin-right:-25px;
    }
.right
    {
    postion:relative;
    left:100px;
    width:25px;
    margin-left:-25px;
    }

And the reworked example code becomes then:

.thetext 
{
    width:400px;
    background:yellow;
    border: 1px dashed red;
    margin:50px;
    padding:5px;
    font-weight:bold;
}
.whiteblob
{
    position:relative;
    top:-140px;
    left:70px;
    width:200px;
    height:50px;
    margin-bottom:-50px;
    border: 4px solid green;
    background:white;
    font-size:2.5em;
    color:red;
    
}
.footerallowedwhitespaceinblue
{
    height:10px;
    background-color:blue;
}
.footer
{
    background-color:grey;
    height:200px;
}

<div class="thetext"><script type="text/javascript">for(c=0;c<50;c++){document.write("Lorem ipsum dolor est, ");}</script>
</div>
<div class="whiteblob">
    &nbsp;buy this!
</div>
<div class="footerallowedwhitespaceinblue">
</div>
<div class="footer">
</div>

http://jsfiddle.net/qqXQn/1/

Here is an example. In this case, the object was moved to the right and then up using a negative top value. Eliminating its trailing margin space required adding an equal negative-margin value.

 position:relative;
 width:310px;
 height:260px;
 top:-332px;
 margin-bottom:-332px;
 left:538px;

You can solve this problem by giving float:left before position:relative. Use margin-left, margin-top properties instead of top, left also.