The |ajouter_class filter adds a class or classes to an HTML tag.
Example
[(#CHEMIN{img/logo.svg)|balise_img|ajouter_class{nav logo-site})]
looks for the img/logo.svg file and generates the image tag with the nav and logo-site classes.
This produces the following HTML:
<img src='squelettes/img/logo.svg?1579179279' alt='' width='200' height='200' class=' nav logo-site' />