fieldset elements and their optional legend elements are structural markup.
In case of the new Special:Preferences page we're outputting
<fieldset id="mw-prefsection-personal-info" class="oo-ui-layout oo-ui-labelElement oo-ui-fieldsetLayout"> <legend class="oo-ui-fieldsetLayout-header"> <span class="oo-ui-iconElement-icon"></span> <span class="oo-ui-labelElement-label">Basisinformationen</span> </legend> <div class="oo-ui-fieldsetLayout-group"> <div aria-disabled="false" class="oo-ui-widget oo-ui-widget-enabled"> <fieldset id="mw-htmlform-info" class="oo-ui-layout oo-ui-fieldsetLayout"> <legend class="oo-ui-fieldsetLayout-header"> <span class="oo-ui-iconElement-icon"></span> <span class="oo-ui-labelElement-label"></span> </legend> <div class="oo-ui-fieldsetLayout-group"> […] </div> </fieldset> </div> </div> </fieldset>
which, especially with the empty legend element in the child fieldset is unnecessary, leads to layout issues and might even result in making screenreaders trip.