one of the documented procedures in this installation of the ACS
ad_check_for_naughty_html   user_submitted_html
What it does:
Returns a human-readable explanation if the user has used any of the HTML tags marked as naughty in the antispam section of ad.ini, empty string otherwise
Defined in: /web/philip/tcl/ad-antispam.tcl

Source code:

    set tag_names [string tolower [ad_parameter_all_values_as_list NaughtyTag antispam]]
    # look for a less than sign, zero or more spaces, then the tag
    if { ! [empty_string_p $tag_names]} { 
        set the_regexp "<\[ \n\t\r\f\]*([join $tag_names "|"])\[ \n\t\r\f>\]"
        if [regexp $the_regexp [string tolower $user_submitted_html]] {
            return "Because of abuse by spammmers, we can't accept submission of any HTML containing any of the following tags:  <code>[join $tag_names " "]</code>"
    # HTML was okay as far as we know