A master page defines common elements across all the pages of your site. You can use the network drive you mapped earlier ...

A master page defines common elements across all the pages of your site. You can use the network drive you mapped earlier to edit your master pages using any HTML editor. To preview your master page, click on its file name or status. While previewing your HTML master page, use the Snippet Gallery to get code snippets for SharePoint functionality that you can copy and paste into your HTML file.