ladislav.soukup

This is simple shortcode to embed responsive iframe to your post. Iframe will hold the specified ratio when page is rescaled.
[rframe ratio=”16×9″ src=”http://flixel.com/player/snqi608mlq1h98w5p9jy” width=”60%” /]
<h4>How it Works</h4>
DIV block with specified width is creted. Inside is transparent PNG file with specifed ratio scaled to 100% width and height set to auto. This image will scale the main DIV to size with correct ratio. 
Embeded IFRAME is then set to fill the wrapper DIV, so it is scaled with the image. 
Image is created on the fly via PHP and inserted as inline image directly to src attribute of img tag.
<h4>Credit</h4>
Thanks to Masau ( http://jsfiddle.net/Masau/7WRHM/ ) for the “trick”.

SB-ResponseFrame

This is simple shortcode to embed responsive (fixed-ratio) iframes

Palasthotel

greatestview

ATTENTION for WP Version 5+: This plugin does not work with Gutenberg (no problems with Classic Editor plugin though)! Gutenberg brings its own CSS for responsive oEmbed elements, so this plugin would double that and create strange effects.
This plugin uses some minimal CSS rules and a wrapping HTML element to maintain the aspect ratio of oEmbed elements with fixed aspect ratio (e. g. YouTube, Vimeo or Soundcloud).
Unlike other plugins, this plugin does not use any JavaScript!
The aspect ratio is calculated from the (iframe, object or embed) HTML tag width and height attributes. An aspect ratio will only be applied, if both width AND height attributes are given by the oEmbed element and if there is no data-secret attribut set (because those are handled via wp-embed.js). Some oEmbeds have no width or height attributes set, because they calculate their dimension via JavaScript. In those cases this plugin has no effect.
You can find a <a href="https://codex.wordpress.org/Embeds#Okay.2C_So_What_Sites_Can_I_Embed_From.3F" rel="nofollow ugc">list of all oEmbed sites supported by WordPress here</a>.

Responsive oEmbed

Exactly what i'm looking for.

Keep up the good work.

Great Plugin

Makes oEmbed elements with fixed aspect ratio (like YouTube, Vimeo or SoundCloud) scale responsively.

SB-ResponseFrame vs Responsive oEmbed