I think You create a control taht will be use at different palaces.
and You want to provide styles to thje control.
If you take
<link href="StyleSheet.css" rel="stylesheet" type="text/css" />
Then you would definitly need styleSheet.css.
So you can use style tag to provide foramatting to your control as
<pre><div style="width:20%; Height=100px">
<asp:CheckBox ID="CheckBox1" runat="server"/>
</div>