Add Embed responsive utility

_includes/nav-css.html

@@ -99,6 +99,7 @@
     <li><a href="#helper-classes-show-hide">Showing and hiding content</a></li>
     <li><a href="#helper-classes-screen-readers">Screen reader content</a></li>
     <li><a href="#helper-classes-image-replacement">Image replacement</a></li>
+    <li><a href="#helper-classes-embeds-responsive">Embeds responsive</a></li>
   </ul>
 </li>
 <li>

css.html

@@ -2575,6 +2575,21 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
 .heading {
   .text-hide();
 }
+{% endhighlight %}
+
+    <h3 id="helper-classes-embeds-responsive">Embeds responsive</h3>
+    <p>Allows browsers to determine videos or slideshow dimensions based on the width of their containing block to create an intrinsic ratio that will properly scale on any device.</p>
+    <p>Rules are directly apply to <code>&lt;iframe&gt;</code>, <code>&lt;embed&gt;</code> and <code>&lt;object&gt;</code> elements, optionally use of an explicit descendant class <code>.embed-responsive-item</code> when you want to match the styling for other attributes.</p>
+{% highlight html %}
+<!-- 16:9 aspect ratio -->
+<div class="embed-responsive embed-responsive-16by9">
+  <iframe class="embed-responsive-item" src="…"></iframe>
+</div>
+
+<!-- 4:3 aspect ratio -->
+<div class="embed-responsive embed-responsive-4by3">
+  <iframe class="embed-responsive-item" src="…"></iframe>
+</div>
 {% endhighlight %}
   </div>
 

dist/css/bootstrap.css

@@ -5615,6 +5615,30 @@ button.close {
 .affix {
   position: fixed;
 }
+.embed-responsive {
+  position: relative;
+  display: block;
+  height: 0;
+  padding: 0;
+  overflow: hidden;
+}
+.embed-responsive .embed-responsive-item,
+.embed-responsive iframe,
+.embed-responsive embed,
+.embed-responsive object {
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+}
+.embed-responsive.embed-responsive-16by9 {
+  padding-bottom: 56.25%;
+}
+.embed-responsive.embed-responsive-4by3 {
+  padding-bottom: 75%;
+}
 @-ms-viewport {
   width: device-width;
 }

less/utilities.less

@@ -54,3 +54,39 @@
 .affix {
   position: fixed;
 }
+
+
+// Embeds responsive
+// -------------------------
+
+// Credit: Nicolas Gallagher
+
+.embed-responsive {
+  position: relative;
+  display: block;
+  height: 0;
+  padding: 0;
+  overflow: hidden;
+
+  .embed-responsive-item,
+  iframe,
+  embed,
+  object {
+    position: absolute;
+    top: 0;
+    left: 0;
+    bottom: 0;
+    height: 100%;
+    width: 100%;
+  }
+
+  // Modifier class for 16:9 aspect ratio
+  &.embed-responsive-16by9 {
+    padding-bottom: 56.25%;
+  }
+
+  // Modifier class for 4:3 aspect ratio
+  &.embed-responsive-4by3 {
+    padding-bottom: 75%;
+  }
+}