I have been doing some masterpage customisations recently and trying to integrate with the OOTB masterpage which Microsoft give you are a pain to work with if you do not know what you are doing. To help others (and help myself in the process) I have taken the latest SharePoint On-Premise and SharePoint Online (Office 365) Seattle.master, properly indented and placed comments around each block to help when you are adding, removing sections from the design. Over the coming weeks I will be adding CSS code snippets to help you add simple CSS Branding to the sites.
OOTB Seattle.Master commented and formatted
Uoooww, amazing effort, will help a lot!