One of the basic things you would want to do with SharePoint Designer 2010 is editing the layout of the page. This seems easy enough, but when SharePoint Designer 2010 blocks access to the yellowish parts of the code it becomes quite impossible. This is for a good reason, and can be annoying at when you first use SharePoint Designer.

To get around this issue you need to click on the Advanced Mode Ribbon button. Then auto-magically it's accessible and you can get the job done! 

After the Advanced Button is clicked, the server does some processing and then it allows access, which is evident when the yellowish background is removed! 

Happy coding!