为什么我的按钮排列

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<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Menu.aspx.cs" Inherits="Menu" %> 
<link rel="stylesheet" href="Styles/Menu.css" type="text/css" /> 

<!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 style="position:absolute; margin: 0px; width:100%; height:100%;"> 
<form id="form1" runat="server"> 
<div style="text-align:center"> 
    <p id="userID">User ID</p> 
</div> 
<br /><br /> 
<div id="content" style="text-align:center; position:relative;"> 
<asp:Button text="MS Walking Scale Test" runat="server" /><br /> 
<asp:Button Text="Get Results" runat="server"/> 
<asp:Button ID="Button1" style="top:80%; position:relative;" Text="LogOut" runat="server" /> 
</div> 
</form> 
</body> 
</html>

Screen shot of Page 为什么我的按钮排列