Why empty space is showing when a div visibilty is false

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ASP.NET

CSS

Hi all i am having 2 check boxes and 2 div's on my web page. On checking particular check box i will show the corresponding div to the user but if i show second div the space which was occupied by first div is occupying empty space why it is happening

Assume my structure is as follows

<div id="div1"  runat="server">
<div>
</div>

<div id="div2" runat="server">
<div>
</div>
</div>

<asp:checkbox id="chk1" runat="server" text="Show Div1" autopostback=true />

<asp:checkbox id="chk2" runat="server" text="Show Div2" autopostback=true />

in code i am showing as follows for both the check boxes

if (chk1.Checked)
        {
            div1.Visible = true;
        }
        else
        {
            div1.Visible = false;
        }

if (chk2.Checked)
        {
            div2.Visible = true;
        }
        else
        {
            div2.Visible = false;
        }

Can any one tell how to resolve this

Sample Style applied for Div is as follows

.style1
      {
          width: 268px;
          height: 19px;
          position: absolute;
          top: 16px;
          left: 335px;
          float: left;
          z-index: 1;
      }