Not saying this is a good way of doing it.
Also all elements within body will be center aligned.
The display type of the table needs to be changed.
body{
text-align:center;
}
table{
display:inline-block;
}
Or div wrapper could be placed around the table like so.
div{
text-align:center;
}
table{
display:inline-block;
}
<div>
<table>
<tr><td><h1>Text Here</h1></td></tr>
<tr><td><h1><asp:textbox id="TextBox1" runat="server" xmlns:asp="#unknown"></asp:textbox></h1></td></tr>
</table>
</div>