Coding with SPIP 3.0
SPIP3.0 Technical Documentation

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#INSERT_HEAD_CSS

#INSERT_HEAD_CSS inserted between the <head> and </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.