<!DOCTYPE html>
<html>
<body>
<form runat="server">
Name: <asp:TextBox id="name" runat="server" />
<br>
Age: <asp:TextBox id="age" runat="server" />
<br><br>
<asp:Button runat="server" Text="Submit" />
<br><br>
<asp:RequiredFieldValidator
ControlToValidate="name"
Text="The name field is required!"
runat="server" />
</form>
</body>
</html>