Apply css class on serverside

 we can apply css class to server controls like controlname.cssclass="xxx" for textboxs,buttons ...
but how we can  apply css class for html controls which are runat server on  serverside
like...
<p runat="server" id='paragraph1" runat="server">
/*Some text */
</p>
how we can  apply css class for paragraph1 on serverside(page load event).
...