When I try to edit the HTML page (for Pageviewer Webpart in MOSS 2007) using the SharePoint designer, I didn’t see the HTML code at all and only see the design! The title bar shows “Contributor Mode” to the end.
To enable this contributor mode,
Go to the site menu and choose contributor settings (Site ==> Contributor Settings)
Then click “Disable Contributor Settings” (Sometimes you have to select “Advanced” button to see this) and click OK. That’s all! Enjoy!!