Add support for JetPack Responsive Videos

[raamdev]

JetPack v3.1 added responsive video support:

First off, we've added responsive video support! One of the things that is often overlooked when developing a responsive theme is making videos responsive. To add support to your theme, just add the following line to your theme's functions.php file.

<?php add_theme_support( 'jetpack-responsive-videos' ); ?>

This ensures that any videos are responsive, no additional markup or special coding required. It just works! Every video on your site will display just as nicely on mobile devices as they do on your largest display.

Independent Publisher needs to be updated to include support for this.

[raamdev]

Closed by PR #126