feat(accordion): do not use static header level for heading

- replace h5 header tag with div tag
- add heading role attribute

Thanks!

src/accordion/accordion.spec.ts

@@ -21,6 +21,10 @@ function getPanelsTitle(element: HTMLElement): HTMLButtonElement[] {
   return <HTMLButtonElement[]>Array.from(element.querySelectorAll('.card > .card-header button'));
 }
 
+function getPanelsHeadings(element: HTMLElement): HTMLDivElement[] {
+  return <HTMLDivElement[]>Array.from(element.querySelectorAll('.card > .card-header [role="heading"]'));
+}
+
 function getButton(element: HTMLElement, index: number): HTMLButtonElement {
   return <HTMLButtonElement>element.querySelectorAll('button[type="button"]')[index];
 }
@@ -202,7 +206,9 @@ describe('ngb-accordion', () => {
     const compiled = fixture.nativeElement;
 
     const titles = getPanelsTitle(compiled);
+    const headings = getPanelsHeadings(compiled);
     expect(titles.length).not.toBe(0);
+    expect(headings.length).not.toBe(0);
 
     titles.forEach((title: HTMLElement, idx: number) => { expect(title.textContent.trim()).toBe(`Panel ${idx + 1}`); });
   });

src/accordion/accordion.ts

@@ -115,13 +115,13 @@ export interface NgbPanelChangeEvent {
     <ng-template ngFor let-panel [ngForOf]="panels">
       <div class="card">
         <div role="tab" id="{{panel.id}}-header" [class]="'card-header ' + (panel.type ? 'bg-'+panel.type: type ? 'bg-'+type : '')">
-          <h5 class="mb-0">
+          <div role="heading">
             <button type="button" class="btn btn-link"
               (click)="toggle(panel.id)" [disabled]="panel.disabled" [class.collapsed]="!panel.isOpen"
               [attr.aria-expanded]="panel.isOpen" [attr.aria-controls]="panel.id">
               {{panel.title}}<ng-template [ngTemplateOutlet]="panel.titleTpl?.templateRef"></ng-template>
             </button>
-          </h5>
+          </div>
         </div>
         <div id="{{panel.id}}" role="tabpanel" [attr.aria-labelledby]="panel.id + '-header'"
              class="collapse" [class.show]="panel.isOpen" *ngIf="!destroyOnHide || panel.isOpen">