For erp5_web it is possible to define form groups following a naming rule "group_id (Group Title)" and "Group Title" is displayed in a legend, according to this comment in form_render.zpt:

It is possible to specify a group id and a group title by naming a group following the "group id (Group Title)" pattern. In this case the group id will be used as fieldset css class and as tag id. The group title will be used as a legend for the fieldset. If no group title is found, we use group id as title.

For historical reasons, the legend was hidden, this feature was only used in erp5_web, but this does not seem bad to enable it in erp5_xhtml_style as well.

At the moment, this is not used much, this script

for sf in context.getPortalObject().portal_skins.objectValues():
  for f in sf.objectValues():
    if not hasattr(f, 'get_groups'):
      continue
    for group in f.get_groups():
      if '(' in group:
        print('/'.join(f.getPhysicalPath()[1:]), group)

return printed

when running on a site with many business template installed yields

erp5/portal_skins/erp5_base/EmbeddedFile_view left (Document Reference)
erp5/portal_skins/erp5_base/EmbeddedFile_view right (Publication Information)
erp5/portal_skins/erp5_base/NotificationMessage_viewEditor left (Page Properties)
erp5/portal_skins/erp5_knowledge_pad/Base_viewKnowledgePadFieldLibrary right column (Toolbox)
erp5/portal_skins/erp5_web/StaticWebSite_viewRedirectAssistConfiguration left (Style)
erp5/portal_skins/erp5_web/WebPage_view left (Page Properties)
erp5/portal_skins/erp5_web/WebPage_view right (Publication)
erp5/portal_skins/erp5_web/WebPage_view bottom (Content)
erp5/portal_skins/erp5_web/WebPage_viewEditor left (Page Properties)
erp5/portal_skins/erp5_web/WebPage_viewFieldLibrary left (Page Properties)
erp5/portal_skins/erp5_web/WebPage_viewFieldLibrary right (Publication)
erp5/portal_skins/erp5_web/WebPage_viewFieldLibrary bottom (Content)
erp5/portal_skins/erp5_web/WebSection_view left (Content)
erp5/portal_skins/erp5_web/WebSection_view right (Layout)
erp5/portal_skins/erp5_web/WebSection_view center (Description)
erp5/portal_skins/erp5_web/WebSection_view bottom (Subsections)
erp5/portal_skins/erp5_web/WebSection_viewDefaultThemeConfiguration left (Style)
erp5/portal_skins/erp5_web/WebSection_viewDefaultThemeConfiguration right (Widgets)
erp5/portal_skins/erp5_web/WebSite_view left (Content)
erp5/portal_skins/erp5_web/WebSite_view right (Layout)
erp5/portal_skins/erp5_web/WebSite_view center (Description)
erp5/portal_skins/erp5_web/WebSite_view bottom (Subsections)
erp5/portal_skins/erp5_web/WebSite_viewRegistrationDialog left (User Login)
erp5/portal_skins/erp5_web/WebSite_viewRegistrationDialog right (User Identity)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout left column transparent (Logo)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout left column (Navigation)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout left column (Sections)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout left column (Contents)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout left column (Search)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout left column (Debug)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout right column (News)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout right column (Toolbox)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_content_layout right column (Highlight)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout left column transparent (Logo)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout left column (Navigation)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout left column (Sections)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout left column (Contents)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout left column (Search)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout left column (Debug)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout right column (News)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout right column (Toolbox)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout right column (Highlight)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_layout right column (Action)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout left column transparent (Logo)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout left column (Navigation)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout left column (Sections)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout left column (Contents)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout left column (Search)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout left column (Debug)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout right column (News)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout right column (Toolbox)
erp5/portal_skins/erp5_web_minimal_theme/erp5_web_section_layout right column (Highlight)
erp5/portal_skins/erp5_dms/Document_viewFieldLibrary left (Document Reference)
erp5/portal_skins/erp5_dms/Document_viewFieldLibrary right (Publication Information)
erp5/portal_skins/erp5_dms/ExternalSource_view left (Document Reference)
erp5/portal_skins/erp5_dms/ExternalSource_view right (Publication Information)
erp5/portal_skins/erp5_dms/OOoDocument_view left (Document Reference)
erp5/portal_skins/erp5_dms/OOoDocument_view right (Publication Information)
erp5/portal_skins/erp5_dms_base/File_view left (Document Reference)
erp5/portal_skins/erp5_dms_base/File_view right (Publication Information)
erp5/portal_skins/erp5_dms_base/Image_view left (Document Reference)
erp5/portal_skins/erp5_dms_base/Image_view right (Publication Information)
erp5/portal_skins/erp5_dms_web/WebIllustration_view left (Document Reference)
erp5/portal_skins/erp5_dms_web/WebIllustration_view right (Publication Information)
erp5/portal_skins/erp5_dms_web/WebIllustration_view bottom (Preview)
erp5/portal_skins/erp5_dms_web/WebPage_view left (Document Reference)
erp5/portal_skins/erp5_dms_web/WebPage_view right (Publication Information)
erp5/portal_skins/erp5_dms_web/WebPage_view bottom (Preview)
erp5/portal_skins/erp5_multimedia/Sound_view left (Document Reference)
erp5/portal_skins/erp5_multimedia/Sound_view right (Publication Information)
erp5/portal_skins/erp5_run_my_doc/Review_view left (Document Reference)
erp5/portal_skins/erp5_run_my_doc/Review_view right (Publication Information)
erp5/portal_skins/erp5_run_my_doc/TestPage_view left (Document Reference)
erp5/portal_skins/erp5_run_my_doc/TestPage_view right (Publication Information)
erp5/portal_skins/erp5_run_my_doc/TestPage_view bottom (Preview)
erp5/portal_skins/erp5_run_my_doc/TestPage_viewFieldLibrary left (Page Properties)
erp5/portal_skins/erp5_run_my_doc/TestPage_viewFieldLibrary right (Publication)
erp5/portal_skins/erp5_run_my_doc/TestPage_viewFieldLibrary bottom (Content)
erp5/portal_skins/erp5_run_my_doc/TestPage_viewTestReport bottom (Content)
erp5/portal_skins/erp5_officejs_slideshow_editor/WebPage_viewForSlideshowEditor bottom (Preview)
erp5/portal_skins/erp5_credential/CredentialRequest_view left (Personal Information)
erp5/portal_skins/erp5_credential/CredentialRequest_view left (Organisation Information)
erp5/portal_skins/erp5_credential/CredentialRequest_view right (Login Information)
erp5/portal_skins/erp5_credential/CredentialUpdate_view left (Personal Information)
erp5/portal_skins/erp5_credential/CredentialUpdate_view left (Organisation Information)
erp5/portal_skins/erp5_credential/CredentialUpdate_view right (Login Information)
erp5/portal_skins/erp5_km/WebSection_viewKMMinimalThemeConfiguration left (Colors)
erp5/portal_skins/erp5_km/WebSection_viewKMMinimalThemeConfiguration right (Static Content)
erp5/portal_skins/erp5_km/WebSection_viewKMMinimalThemeConfiguration left (Navigation)
erp5/portal_skins/erp5_km/WebSection_viewKMMinimalThemeConfiguration right (Gadgets)
erp5/portal_skins/erp5_km_theme/Base_viewKMFieldLibrary right column (Toolbox)
erp5/portal_skins/erp5_km_theme/WebSection_viewMinimalKMWidgetFieldLibrary bottom gadget (Bottom Gadget)
erp5/portal_skins/erp5_km_theme/WebSection_viewMinimalKMWidgetFieldLibrary right column gadget (Right Gadget)
erp5/portal_skins/erp5_km_theme/erp5_km_minimal_content_layout bottom gadget (Bottom Gadget)
erp5/portal_skins/erp5_km_theme/erp5_km_minimal_content_layout right column gadget (Right Gadget)
erp5/portal_skins/erp5_km_theme/erp5_km_minimal_layout bottom gadget (Bottom Gadget)
erp5/portal_skins/erp5_km_theme/erp5_km_minimal_layout right column gadget (Right Gadget)
erp5/portal_skins/erp5_km_theme/login_form left registration (Login)
erp5/portal_skins/erp5_km_theme/login_form right registration (New User Registration)

and this does not look bad when viewing the forms.