The #INSERT_HEAD tag must be included between the <head> and </head> tags in your template files. It enables SPIP and its extensions, as well as whatever plugins you use, to add content between these two HTML tags.
There has been a compression function included. This functionality optimises the usage of available bandwidth for all of the elements passed with this tag, be removing any and all "superfluous" spaces and comments from the .js and .css files for all of the templates. This functionality can be enabled or disabled from within the configuration panel in the site’s private zone.
It has been highly recommended that you systematically use this tag in your template files. This recommendation is because some certain core functionalities in SPIP have been transferred into extensions that now behave exactly like normal plugins.