I found an interesting thing about the error message I was getting with SharePoint Designer. The error is:
“Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Windows SharePoint Services-compatible HTML editor such as FrontPage. If the problem persists, contact your Web server administrator”
I had a complicated Data view web part that I did not want to recreate.
Open the aspx file in notepad from SharePoint Designer. If you notice a bunch of HTML markup formatted as “>” etc, then open the file normally in SharePoint designer Code View. Copy ALL the code and paste it into Notepad. Copy the code from notepad and replace your code view text. Save and view in browser. No more XSL DATA VIEW ERRORS!
The code view actually contains a rich text type of rendering. When you copy it out and paste it in notepad, it loses all of the special formatting. This allows the page to render correctly.