style="position: absolute;top: 8%;left: 12%;">
<form  onsubmit="return register(this);">
    <h1>欢迎注册网上商城</h1>
    <table>
        <tr>
            <td colspan="2" id="alert" class="tds" style="text-align: center;color: red;"></td>
        </tr>
        <tr>
            <td class="tds"><label for="username">用户名：</label></td>
            <td>
                <input type="text" name="username" id="username" onblur="usernameBlur(this)"/>
                <span style="color: red" id="user