Master Page Weirdness - "Content controls have to be top-level controls in a content page or a nested master page that references a master page."
Your web form shouldn't have all of that markup (like the <html> tag). Since it has a master page, you just start with the content tag. Your aspx page should look like this:
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Default.aspx.vb" Inherits="WebUI._Default" MasterPageFile="~/Site1.Master" %>
<asp:content id="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
This is the body!
</asp:content>
When you're adding a new aspx page make sure to check "select master page" in the "add new item" dialog.
Here's another way using Visual Studio: If you do New Item in Visual Studio and you select Web Form, it will create a standalone *.aspx web form, which is what you have for your current web form (is this what you did?). You need to select Web Content Form and then select the master page you want attached to it.
Your web form should look like this:
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Default.aspx.vb" Inherits="WebUI._Default" MasterPageFile="~/Site1.Master" %>
<asp:Content runat="server" ID="head" ContentPlaceHolderId="head">
<!-- stuff you want in >head%lt; -->
</asp:Content>
<asp:Content runat="server" ID="content" ContentPlaceHolderId="ContentPlaceHolder1">
<h1>Your content</h1>
</asp:Content>
Note that there is no <html> tag