#INSERT_HEAD_CSS inserted between the
</head> HTML Tags enables plugins to add CSS scripts by using the insert_head_css pipeline. If this tag does not exist in the template file, then
#INSERT_HEAD will add the contents of the pipeline itself.
In SPIP’s default template files, this tag is inserted just before the
habillage.css CSS file in squelettes-dist/inc-head.html. This means that graphical themes that overwrite this
habillage.css file can likewise be overwritten themselves, using CSS directives or include references, by declarations added by the corresponding installed plugins.