Assorted accessibility (and some consistency) fixes for documentation

Closes #14951 by merging it.

docs/_includes/css/buttons.html

@@ -35,6 +35,11 @@
 <button type="button" class="btn btn-link">Link</button>
 {% endhighlight %}
 
+  <div class="bs-callout bs-callout-warning">
+    <h4>Conveying meaning to assistive technologies</h4>
+    <p>Using color to add meaning to a button only provides a visual indication, which will not be conveyed to users of assistive technologies – such as screen readers. Ensure that information denoted by the color is either obvious from the content itself (the visible text of the button), or is included through alternative means, such as additional text hidden with the <code>sr-only</code> class.</p>
+  </div>
+
   <h2 id="buttons-sizes">Sizes</h2>
   <p>Fancy larger or smaller buttons? Add <code>.btn-lg</code>, <code>.btn-sm</code>, or <code>.btn-xs</code> for additional sizes.</p>
   <div class="bs-example">
@@ -88,7 +93,7 @@
 
 
   <h2 id="buttons-active">Active state</h2>
-  <p>Buttons will appear pressed (with a darker background, darker border, and inset shadow) when active. For <code>&lt;button&gt;</code> elements, this is done via <code>:active</code>. For <code>&lt;a&gt;</code> elements, it's done with <code>.active</code>. However, you may use <code>.active</code> on <code>&lt;button&gt;</code>s should you need to replicate the active state programmatically.</p>
+  <p>Buttons will appear pressed (with a darker background, darker border, and inset shadow) when active. For <code>&lt;button&gt;</code> elements, this is done via <code>:active</code>. For <code>&lt;a&gt;</code> elements, it's done with <code>.active</code>. However, you may use <code>.active</code> on <code>&lt;button&gt;</code>s (and include the <code>aria-pressed="true"</code> attribute) should you need to replicate the active state programmatically.</p>
 
   <h3>Button element</h3>
   <p>No need to add <code>:active</code> as it's a pseudo-class, but if you need to force the same appearance, go ahead and add <code>.active</code>.</p>
@@ -147,7 +152,7 @@
   </p>
   <div class="bs-callout bs-callout-warning">
     <h4>Link functionality caveat</h4>
-    <p>This class uses <code>pointer-events: none</code> to try to disable the link functionality of <code>&lt;a&gt;</code>s, but that CSS property is not yet standardized and isn't fully supported in Opera 18 and below, or in Internet Explorer 11. So to be safe, use custom JavaScript to disable such links.</p>
+    <p>This class uses <code>pointer-events: none</code> to try to disable the link functionality of <code>&lt;a&gt;</code>s, but that CSS property is not yet standardized and isn't fully supported in Opera 18 and below, or in Internet Explorer 11. In addition, even in browsers that do support <code>pointer-events: none</code>, keyboard navigation remains unaffected, meaning that sighted keyboard users and users of assistive technologies will still be able to activate these links. So to be safe, use custom JavaScript to disable such links.</p>
   </div>
   <div class="bs-callout bs-callout-warning">
     <h4>Context-specific usage</h4>
@@ -170,6 +175,11 @@
 <input class="btn btn-default" type="submit" value="Submit">
 {% endhighlight %}
 
+  <div class="bs-callout bs-callout-warning">
+    <h4>Links acting as buttons</h4>
+    <p>If the <code>&lt;a&gt;</code> elements are used to act as buttons – triggering in-page functionality, rather than navigating to another document or section within the current page – they should also be given an appropriate <code>role="button"</code>.</p>
+  </div>
+
   <div class="bs-callout bs-callout-warning">
     <h4>Cross-browser rendering</h4>
     <p>As a best practice, <strong>we highly recommend using the <code>&lt;button&gt;</code> element whenever possible</strong> to ensure matching cross-browser rendering.</p>