I want to customize some aspx / ascx files of a precompiled site (the
project portofolio web access site). I have no access to source code.

Using Visual Studio, when I try to edit one these files in "design view", I
get the following two categories of errors:

1)    Some "Cannot switch views" errors, like the following:

"Cannot switch views: This end tag has no matching start tag."
"Cannot switch views: Validation (XHTML 1.0 Transitional): Element '
myElement ' cannot be nested within element 'body'."
"Cannot switch views: Validation (XHTML 1.0 Transitional): Attribute 'align'
can be specified only once in a tag."

2)    MANY "Element 'myElement' is not a known element.." errors, as:

"Element 'Benefits' is not a known element. This can occur if there is a
compilation error in the Web site"

In relation to registered user controls, like:
.
<%@ Register TagPrefix="WP" TagName="Schedule"
Src="../Parts/Projects/Schedule.ascx" %>
.

The web site I´m trying to customize is the Project Portfolio 2007 Web
Access (fresh install and working properly).

Using VS 2005 SP1, opening the site by using the "Open site" from "file"
menu.