Hi yuenli,
Can you reproduce this issue by the example page below? Is some test pages online? So we can check it for you.
<%@ Page Language="C#" AutoEventWireup="true" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<%@ Register TagPrefix="CE" Namespace="CuteEditor" Assembly="CuteEditor" %>
<html>
<head>
<title>example</title>
</head>
<body>
<form id="Form1" runat="server">
<CE:Editor ID="Editor1" runat="server">
</CE:Editor>
</form>
</body>
</html>
Regards,
ken