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The 21:9 aspect ratio class

A new aspect ratio class has been added to Bootstrap 4: the .embed-responsive-21by9 class for a 21:9 ratio. Here is an example of it:

<div class="embed-responsive embed-responsive-21by9">
                <iframe class="embed-responsive-item" src="http://packtpub.com"></iframe>
            </div>

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