Fix settings heading hierarchy: do not use h3 for description

as it distorts the outline which is detrimental to accessibility;
instead use the `<header>` pattern as recommended by W3C in:
https://www.w3.org/TR/2017/WD-html52-20170228/common-idioms-without-dedicated-elements.html

Also, do not rely on style the caret icon by element but by class name
(using this pattern reduces cognitive burden and improves performance).

packages/settingeditor/src/SettingsFormEditor.tsx

@@ -311,11 +311,17 @@ export class SettingsFormEditor extends React.Component<
             this.props.onSelect(this.props.settings.id);
           }}
         >
-          <div className="jp-SettingsTitle">
-            <icon.react tag="span" elementPosition="center" />
-            <h2> {this.props.settings.schema.title} </h2>
-            <h3> {this.props.settings.schema.description} </h3>
-          </div>
+          <header className="jp-SettingsTitle">
+            <icon.react
+              tag="span"
+              elementPosition="center"
+              className="jp-SettingsTitle-caret"
+            />
+            <h2>{this.props.settings.schema.title}</h2>
+            <div className="jp-SettingsHeader-description">
+              {this.props.settings.schema.description}
+            </div>
+          </header>
           {this.state.isModified && (
             <button className="jp-RestoreButton" onClick={this.reset}>
               {trans.__('Restore to Defaults')}

packages/settingeditor/style/base.css

@@ -353,7 +353,7 @@ ul.jp-PluginList li.jp-mod-selected span.jp-PluginList-icon.jp-FileIcon {
   margin: 1em;
 }
 
-.jp-SettingsPanel .jp-SettingsHeader h3 {
+.jp-SettingsPanel .jp-SettingsHeader-description {
   font-size: var(--jp-content-font-size2);
   color: var(--jp-ui-font-color1);
   font-weight: 200;
@@ -381,7 +381,7 @@ ul.jp-PluginList li.jp-mod-selected span.jp-PluginList-icon.jp-FileIcon {
   padding-left: 1em;
 }
 
-.jp-SettingsPanel .jp-SettingsTitle span {
+.jp-SettingsPanel .jp-SettingsTitle-caret {
   width: 2em;
 }