It should be fairly easy to write an html validation for the form builder / ad_form so that you could do formwidget:html(text), but no one has done so yet.
Search in your /api-doc/ for "template::data::validate::" for a list of all datatypes that you can use.
Until someone adds it, you can try to cut the validation block workaround from forums and make it work in your ad_form somehow:
element create message content \
-label [_ forums.Body] \
-datatype text \
-widget textarea \
-html {rows 20 cols 60 wrap soft} \
-validate {
empty {expr ![empty_string_p [string trim $value]]} { [_ forums.lt_Please_enter_a_messag] }
html { expr {( [string match [set l_html_p [ns_queryget html_p f]] "t"] && [empty_string_p [set v_message [ad_quotehtml [ad_html_security_check $value]]]] ) || [string match $l_html_p "f"] } }
{}
}
