Nicolas- The new HTML exporter does not actually use the value of the :HTML_CONTAINER_CLASS: property. The old exporter saved the value off into the property :html-container-class on the headline (with `org-export-remember-html-container-classes'), the new exporter references the `:html-container-class' property on the headline, but i see no way to set it.
My guess is that on ox-html.el:1917 (the only reference to :html-container-class) (extra-class (org-element-property :html-container-class headline)) should be (extra-class (org-element-property :HTML_CONTAINER_CLASS headline)) Does this seem the correct fix to you? rick