When editing a Master Page in sharePoint 2013;
migrating custom master page into SharePoint 2013,
When editing a Master Page in sharePoint 2013
When we try to modify a SharePoint 2013 Master Page, we need to edit the Master Page’s .html file only. Once saved, the associated master page file gets updated automatically.
So don’t attempt to modify the .master as it should be perfectly in sync with its html counterpart.
When editing a Layout Page –
When we create a Custom Layout Page using Design Manager, two files – .aspx and .html gets created in the Master page gallery.
If we need to make any Changes to the Layout page, make sure you download the .html file of our layout page and make Changes to it. The .aspx page will get updated automatically.
Also, don’t forget to publish the .html or .aspx as a major version.
When we migrate the SharePoint 2010 site that have custom master page with custom control and webpart associated with this, and we want the SharePoint 2013 master page’s look and feel in this case we need to copy the SharePoint 2013 master page like we did the copy of seattle.master
And added our custom control’s reference and added our custom css references in it than only we can get the both our custom branding and SharePoint 2013 look and feel in our SharePoint 2010 migrated site in SP 2013.