Html label tag and ASP.NET

<label for="Username">Username:</label>
<input type="text" id="Username" runat="server" />

<label>
    <span>Username</span>
    <input type="text" id="Username" runat="server" />
</label>

<asp:Label ID="UsernameLabel"
           Text="Username:"
           AssociatedControlID="UsernameTextBox"
           runat="server">
    <asp:TextBox ID="UsernameTextBox" runat="server" />
</asp:Label>

<asp:Label ID="label1" runat="server" Text="Username" AssociatedControlID="Text1" />
<asp:TextBox ID="Text1" runat="server" />

<asp:Label AssociatedControlID="Username" runat="server" Text="Username:"></asp:Label>
<asp:TextBox ID="Username" runat="server"></asp:TextBox>

<label for="<%= Username.ClientID %>">Username:</label>
<asp:TextBox ID="Username" runat="server" />

<p><asp:Label ID="label1"           Text="Username:"           AssociatedControlID="txtUserName"           runat="server">    <asp:TextBox ID="txtUserName" runat="server" /></asp:Label></p>

<label for="Username">Username:</label>
<asp:TextBox ID="Username" runat="server" ClientIDMode="Static" />