Add [hidden] selector to Base to counteract all display declarations

Anytime a Pure style rule uses the `display` declaration with a value
other than `none`, using the `hidden` HTML attribute on that element
won't have the desired affect of hiding that element. To counteract
this, A new rule has been added to Base to force
`display: none !important;` on elements that use the `hidden` HTML
attribute.

The following button will be hidden from view with these changes:

    <button class="pure-button" hidden>No showy</button>

Fixes #177

HISTORY.md

@@ -6,8 +6,17 @@ NEXT
 
 ### Base
 
-* The `body` selector in the `base-context` module is now replaced with `.pure`,
-  like the `html` selector.
+* Elements that have Pure classnames which set a `display` declaration _and_ use
+  the `hidden` HTML attribute will now properly be hidden. With these changes,
+  the following button will be hidden from view:
+
+    ```html
+    <button class="pure-button" hidden>No showy</button>
+    ```
+
+  A new rule for the `[hidden]` selector has been added with the declaration:
+  `display: none !important;`. This is a time where it's appropriate for a
+  project like Pure to use `!important`. ([#177][])
 
 ### Buttons
 
@@ -47,6 +56,7 @@ NEXT
 [#144]: https://github.com/yui/pure/issues/144
 [#164]: https://github.com/yui/pure/issues/164
 [#174]: https://github.com/yui/pure/issues/174
+[#177]: https://github.com/yui/pure/issues/177
 [#191]: https://github.com/yui/pure/issues/191
 [#200]: https://github.com/yui/pure/issues/200
 [#221]: https://github.com/yui/pure/issues/221

src/base/css/base.css

@@ -7,3 +7,11 @@
 /**
  * Extra rules that Pure adds on top of Normalize.css
  */
+
+/**
+ * Always hide an element when it has the `hidden` HTML attribute.
+ */
+
+[hidden] {
+    display: none !important;
+}