The insert_head_css
pipeline is used by plugins to insert the CSS files that they need to operate correctly into the section of the SPIP template that includes the #INSERT_HEAD_CSS
tag if there is one, and if not then at the start of the code included using the #INSERT_HEAD
tag. This allows a template to indicate a specific location for additionally loaded CSS code.
It is called quite simply by using:
return pipeline('insert_head_css', '');