Release v4.3.0 (#28228)v4.3.0

* Prep for v4.3.0

* More updates for v4.3.0.

* rerun to build docs source map

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+---
+layout: docs
+title: Embeds
+description: Create responsive video or slideshow embeds based on the width of the parent by creating an intrinsic ratio that scales on any device.
+group: utilities
+toc: true
+---
+
+## About
+
+Rules are directly applied to `<iframe>`, `<embed>`, `<video>`, and `<object>` elements; optionally use an explicit descendant class `.embed-responsive-item` when you want to match the styling for other attributes.
+
+**Pro-Tip!** You don't need to include `frameborder="0"` in your `<iframe>`s as we override that for you.
+
+## Example
+
+Wrap any embed like an `<iframe>` in a parent element with `.embed-responsive` and an aspect ratio. The `.embed-responsive-item` isn't strictly required, but we encourage it.
+
+{% capture example %}
+<div class="embed-responsive embed-responsive-16by9">
+  <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen></iframe>
+</div>
+{% endcapture %}
+{% include example.html content=example %}
+
+## Aspect ratios
+
+Aspect ratios can be customized with modifier classes. By default the following ratio classes are provided:
+
+{% highlight html %}
+<!-- 21:9 aspect ratio -->
+<div class="embed-responsive embed-responsive-21by9">
+  <iframe class="embed-responsive-item" src="..."></iframe>
+</div>
+
+<!-- 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>
+
+<!-- 1:1 aspect ratio -->
+<div class="embed-responsive embed-responsive-1by1">
+  <iframe class="embed-responsive-item" src="..."></iframe>
+</div>
+{% endhighlight %}
+
+Within `_variables.scss`, you can change the aspect ratios you want to use. Here's an example of the `$embed-responsive-aspect-ratios` list:
+
+{% highlight scss %}
+$embed-responsive-aspect-ratios: (
+  (21 9),
+  (16 9),
+  (4 3),
+  (1 1)
+) !default;
+{% endhighlight %}