How to change CSS class of a HTML page element using ASP.NET?

<li id="li1" class="class1">
<li id="li2" class="class1">
<li id="li3" class="class1">

li1.className="class2"

WebControl control = (WebControl)FindControl("li1");
control.CssClass="class2";

<li runat="server ... ></li>

System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append("<script language="javascript">");
sb.Append("document.getElementById("li1").className="newClass";")
sb.Append("</script>");
ClientScript.RegisterStartupScript(this.GetType(), "MyScript", sb.ToString());

<li id="li1" runat="server">stuff</li>

li1.Attributes["Class"] = "class1";
li2.Attributes["Class"] = "class2";

using System.Web.UI.HtmlControls;

HtmlGenericControl liItem = (HtmlGenericControl) ctl.FindControl("liItemID");
liItem.Attributes.Add("class", "someCssClass");