Styling and accessibility clarification for navs documentation

* Split out/expand mention that base navs lack active styles
* Add accessibility notes for navigation bars vs dynamic tabbed interfaces

docs/components/navs.md

@@ -14,7 +14,11 @@ Navigation available in Bootstrap share general markup and styles, from the base
 
 ## Base nav
 
-The base `.nav` component is built with flexbox and provide a strong foundation for building all types of navigation components. It includes some style overrides (for working with lists), some link padding for larger hit areas, and basic disabled styling. No active states are included in the base nav.
+The base `.nav` component is built with flexbox and provide a strong foundation for building all types of navigation components. It includes some style overrides (for working with lists), some link padding for larger hit areas, and basic disabled styling.
+
+{% callout info %}
+The base `.nav` component does not include any `.active` state. The following examples include the class, mainly to demonstrate that this particular class does not trigger any special styling.
+{% endcallout %}
 
 {% example html %}
 <ul class="nav">
@@ -235,6 +239,8 @@ If you need responsive nav variations, consider using a series of [flexbox utili
 
 If you're using navs to provide a navigation bar, be sure to add a `role="navigation"` to the most logical parent container of the `<ul>`, or wrap a `<nav>` element around the whole navigation. Do not add the role to the `<ul>` itself, as this would prevent it from being announced as an actual list by assistive technologies.
 
+Note that navigation bars, even if visually styled as tabs with the `.nav-tabs` class, should **not** be given `role="tablist"`, `role="tab"` or `role="tabpanel"` attributes. These are only appropriate for dynamic tabbed interfaces, as described in the [<abbr title="Web Accessibility Initiative">WAI</abbr> <abbr title="Accessible Rich Internet Applications">ARIA</abbr> Authoring Practices](https://www.w3.org/TR/wai-aria-practices/#tabpanel). See [JavaScript behavior for dynamic tabbed interfaces](#javascript-behavior-for-dynamic-tabbed-interfaces) in this section for an example.
+
 ## Using dropdowns
 
 Add dropdown menus with a little extra HTML and the [dropdowns JavaScript plugin]({{ site.baseurl }}/components/dropdowns/#usage).
@@ -291,10 +297,12 @@ Add dropdown menus with a little extra HTML and the [dropdowns JavaScript plugin
 </ul>
 {% endexample %}
 
-## JavaScript behavior
+## JavaScript behavior for dynamic tabbed interfaces
 
 Use the tab JavaScript plugin—include it individually or through the compiled `bootstrap.js` file—to extend our navigational tabs and pills to create tabbable panes of local content, even via dropdown menus.
 
+Dynamic tabbed interfaces, as described in the [<abbr title="Web Accessibility Initiative">WAI</abbr> <abbr title="Accessible Rich Internet Applications">ARIA</abbr> Authoring Practices](https://www.w3.org/TR/wai-aria-practices/#tabpanel), require `role="tablist"`, `role="tab"`, `role="tabpanel"`, and additional `aria-` attributes in order to convey their structure, functionality and current state to users of assistive technologies (such as screen readers).
+
 <div class="bd-example bd-example-tabs">
   <ul class="nav nav-tabs" id="myTab" role="tablist">
     <li class="nav-item">