Correcting the Markup not valid Master Page error in SharePoint Designer 2007
"This page contains markup that is not valid when attached to a Master Page. Correct Problem in code view."
This issue can occur when opening a page within SharePoint designer 2007, instead of the page rendering in design view, you get the above error. The quickest fix for this I have found is:
1. Open the page with the error in "code view".
2. Click the "Format" menu from the top menu bar.
3. Choose "Master Page" and "Attach Master Page".
4. Select an alternate master page to the one currently selected, or select a master page if one isn't.
5. Click OK.
That will update the master page link in your page and should allow it to render in design view.