SharePoint Designer was giving me a popup to tell me I could edit the page in advanced mode. This has stopped happening and now I just get a page with yellow background.
How can I edit the page layout in advanced mode?
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Sign up to join this communityHave a look in the ribbon and click Advanced Mode.
More info in this article - http://blogs.microsoft.co.il/blogs/cizi/archive/2010/08/15/editing-page-in-sharepoint-designer-2010-with-yellow-marked-code-advance-mode.aspx
In answer to @Havoc
If advanced mode not available it may be due to a setting in Central Admin From this link
Central Administration
Use the SharePoint Designer Settings page in Central Administration to manage SharePoint Designer at the web application level, which includes all site collections in the web application. These settings apply to site collection administrators and all users of the web application.
Enable Detaching Pages from the Site Definition
Enables or disables the ability to detach pages from the site definition. If unchecked, SharePoint Designer only allows you to edit pages in normal mode. The Edit File in Advanced Mode is disabled. Note This setting does not apply to pages that have already been detached as well as new blank ASPX or HTML pages created by the user.
To elaborate Paul Rowland's solution
1. Open https://youraccount.sharepoint.com/_layouts/15/SharePointDesignerSettings.aspx
Then turn on [Enable Detaching Pages from the Site Definition] as above picture shows.
Then open your team site: like
https://yoursite.sharepoint.com/teams/Marketing/_layouts/15/settings.aspx