\-commands are executed by mailman/1 from print_html/1 or html_print_length/2. These commands are called in the calling context of the html_post//2 call.
A typical usage scenario is to get required CSS links in the document head in a reusable fashion. First, we define css//1 as:
css(URL) --> html_post(css, link([ type('text/css'), rel('stylesheet'), href(URL) ])).
Next we insert the unique CSS links, in the pagehead using the following call to reply_html_page/2:
reply_html_page([ title(...), \html_receive(css) ], ...)