Still space between elements (buttons) with "margin: 0"?
Just remove any spaces between DIVs:
<div class="redline_changes">
<a href="#">Left Button</a>
</div><div class="redline_comments">
<a href="#">Right Button</a>
</div>
The two buttons have a space between them because of the return and the indentation you used to format your HTML.
Just float both containers: http://jsfiddle.net/TRVfh/19/
.redline_changes, .redline_comments{
margin-left: 0px;
margin-right: 0px; float: left;
}
The space is caused by the newline and indentation between the two internal divs. Removing the white space will bring the two closer.
<div class="shell" style="width: 950px; ">
<div class="redline_changes">
<a href="#">Left Button</a>
</div><div class="redline_comments">
<a href="#">Right Button</a>
</div>
</div>