Use a header element for header template

bakerydemo/templates/includes/header.html

@@ -1,6 +1,6 @@
 {% load navigation_tags %}
 
-<div class="header clearfix">
+<header class="header clearfix">
     <div class="container">
         <div class="navigation" data-navigation>
             <a href="/" class="navigation__logo">The Wagtail Bakery</a>
@@ -54,4 +54,4 @@
             </form>
         </div>
     </div>
-</div>
+</header>