Improve accessibility of navigation menu button

Add `type` and label text.

Fixes #1099

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Add note about Staticman GitHub compatibility. [#1273](https://github.com/mmistakes/minimal-mistakes/issues/1273)
 - Add missing Brazilian Portuguese translations to `ui-text.yml`. [#1278](https://github.com/mmistakes/minimal-mistakes/pull/1278)
 - Update font stack documentation. [#1292](https://github.com/mmistakes/minimal-mistakes/pull/1292)
+- Improve accessibility of navigation menu button. [#1099](https://github.com/mmistakes/minimal-mistakes/issues/1099)
 
 ### Bug Fixes
 

_includes/masthead.html

@@ -13,7 +13,10 @@
             <li class="masthead__menu-item"><a href="{{ domain }}{{ link.url }}">{{ link.title }}</a></li>
           {% endfor %}
         </ul>
-        <button><div class="navicon"></div></button>
+        <button type="button">
+          <span class="visually-hidden">{{ site.data.ui-text[site.locale].menu_label | default: "Toggle Menu" }}</span>
+          <div class="navicon"></div>
+        </button>
         <ul class="hidden-links hidden"></ul>
       </nav>
     </div>

docs/_docs/18-history.md

@@ -4,7 +4,7 @@ permalink: /docs/history/
 excerpt: "Change log of enhancements and bug fixes made to the theme."
 sidebar:
   nav: docs
-last_modified_at: 2017-10-04T08:07:18-04:00
+last_modified_at: 2017-10-04T11:06:15-04:00
 ---
 
 ## Unreleased
@@ -16,6 +16,7 @@ last_modified_at: 2017-10-04T08:07:18-04:00
 - Add note about Staticman GitHub compatibility. [#1273](https://github.com/mmistakes/minimal-mistakes/issues/1273)
 - Add missing Brazilian Portuguese translations to `ui-text.yml`. [#1278](https://github.com/mmistakes/minimal-mistakes/pull/1278)
 - Update font stack documentation. [#1292](https://github.com/mmistakes/minimal-mistakes/pull/1292)
+- Improve accessibility of navigation menu button. [#1099](https://github.com/mmistakes/minimal-mistakes/issues/1099)
 
 ### Bug Fixes
 

docs/_includes/masthead.html

@@ -13,9 +13,12 @@
               <li class="masthead__menu-item"><a href="{{ domain }}{{ link.url }}">{{ link.title }}</a></li>
             {% endfor %}
           </ul>
-        <button><div class="navicon"></div></button>
+          <button type="button">
+            <span class="visually-hidden">{{ site.data.ui-text[site.locale].menu_label | default: "Toggle Menu" }}</span>
+            <div class="navicon"></div>
+          </button>
           <ul class="hidden-links hidden"></ul>
         </nav>
       </div>
     </div>
-</div>
+  </div>