Button postback problem

I everybody, this is weird and I can't find the answer in google, maybe because is too simple.
I'm using VS2005/ASP.NET 2.0.  I have a webform with just one button like this:
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_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>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    
        <asp:Button ID="Button1" runat="server" Style="z-index: 100; left: 408px; position: absolut ...