From Joomla! Documentation
JDocumentRendererHead is responsible for rendering the output for the <head> section of an HTML document. It is called whenever a
<jdoc:include type="head" /> statement is encountered in the document template.
Note that any internal stylesheet rules that may have been added by the JDocument/addStyleDeclaration method will be output after all the external stylesheet links (both the default ones and any that may have been added by calls to the JDocument/addStyleSheet method. This ensures that the correct stylesheet cascade is preserved.