diff -u b/core/modules/system/templates/fieldset.html.twig b/core/modules/system/templates/fieldset.html.twig
--- b/core/modules/system/templates/fieldset.html.twig
+++ b/core/modules/system/templates/fieldset.html.twig
@@ -21,21 +21,26 @@
  * @ingroup themeable
  */
 #}
-<fieldset{{ attributes }}>
+<fieldset{{ attributes.addClass('form-wrapper') }}>
+  {%
+    set legend_span_classes = [
+      required ? 'form-required',
+    ]
+  %}
   {#  Always wrap fieldset legends in a SPAN for CSS positioning. #}
   <legend{{ legend.attributes }}>
-    <span{{ legend_span.attributes }}>{{ legend.title }}</span>
+    <span{{ legend_span.attributes.addClass(legend_span_classes) }}>{{ legend.title }}</span>
   </legend>
-  <div>
+  <div class="fieldset-wrapper">
     {% if prefix %}
-      <span>{{ prefix }}</span>
+      <span class="field-prefix">{{ prefix }}</span>
     {% endif %}
     {{ children }}
     {% if suffix %}
-      <span>{{ suffix }}</span>
+      <span class="field-suffix">{{ suffix }}</span>
     {% endif %}
     {% if description.content %}
-      <div{{ description.attributes }}>{{ description.content }}</div>
+      <div{{ description.attributes.addClass('description') }}>{{ description.content }}</div>
     {% endif %}
   </div>
 </fieldset>
only in patch2:
unchanged:
--- a/core/themes/classy/templates/system/fieldset.html.twig
+++ b/core/themes/classy/templates/system/fieldset.html.twig
@@ -21,10 +21,9 @@
  * @ingroup themeable
  */
 #}
-<fieldset{{ attributes.addClass('form-item', 'form-wrapper') }}>
+<fieldset{{ attributes.addClass('form-wrapper') }}>
   {%
     set legend_span_classes = [
-      'fieldset-legend',
       required ? 'form-required',
     ]
   %}
@@ -32,7 +31,7 @@
   <legend{{ legend.attributes }}>
     <span{{ legend_span.attributes.addClass(legend_span_classes) }}>{{ legend.title }}</span>
   </legend>
-  <div class="fieldset-wrapper">
+  <div>
     {% if prefix %}
       <span class="field-prefix">{{ prefix }}</span>
     {% endif %}