Can't get div to use class

I have a label like so
 
<asp:label id="tab" runat="server"></asp:label> With vb like this tab.Text &= "<div class='tab'>" & reader(1) & "</div>"  
The problem is, it's not picking up any of my css. I.E. the width and height. It picks up the color and border. Any ideas why?
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