In order for a page to function as a SharePoint master page, it must contain the Page Head Contents component in its head. ...

In order for a page to function as a SharePoint master page, it must contain the Page Head Contents component in its head.  The component includes several important pieces of functionality, such as the CSS Link and the Additional Page Head content placeholder.

The CSS Link holds CSS files commonly referenced by SharePoint components. Any CSS references emitted by a component should be placed here.

The Additional Page Head content placeholder allows page layouts to define content that should be placed in the head section of the page. For example, if a page layout defines its own CSS references, they should be placed here.