Visual Studio 2008 w/Sp1
To reproduce my problem I simply create a new .Net 2.0 web application and add a page with the following markup:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication5._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:button id="button1" runat="server" />
</div>
</form>
</body>
</html>
what happens is that a line is drawn under :button with the statement "Validation (): Element 'button' is not supported."
I've tried repairing the .net framework; however, that had no impact. This started happening on two different machines, one is vista the other is XP within the past week.
UPDATE: I closed this question because after spending 2 days trying to fix it I gave up and performed a complete reinstall of Visual Studio 2008. This resolved whatever was jacked up and now the machines in question work properly. I upvoted each of you for the help.