Left container problem on master page

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C#

ASP.NET

CSS

Hi Guys i need a help about my master page.

i am creating Left Container on my master page. and i am using a user control
name is LeftnavigationControl.ascx.
and it's css file is

#mainBody #LeftContainer{width:235px;margin-left:3px;  float:left; margin-right:-100%; position:relative; background:url(images/leftBoxbg.jpg) repeat-y top right; }
#mainBody #LeftContainer .leftBoxContent{ padding:15px 0px  8px 0px; margin:0 0 0 12px; width:200px; background:url(images/bottomSlider.jpg) no-repeat bottom left; color:#FFFFFF;}

And in the next step

i am using div on my master page.inside the body tag there is a div tag which is

<div id="mainBody">
<div id="LeftContainer">
<div class="leftBoxContent"></div>
<LeftnavigationControl:LeftnavigationControl ID="LeftnavigationControl1" runat="server" />
</div>

But when i run the application so main left container is not displaying please help me