HTML mode provides a mechanism for specifying the body of a page using HTML. This can be useful for including features that are not provided by the other Carrelet modes. The HTML is inserted into the page template, so the normal page 'furniture' is maintained.
Note that Carrelet makes no attempt to check the validity of the HTML it is passed,so it is perfectly possible to render the page unusable. In general, HTML should be used only by those with a good working understanding of the language, and, even then, with caution.
If the page you want to produce is fairly similar to a normal Carrelet page, consider using the HTML convertor (Cliquez ici)