Vimeo & YouTube Video

Vimeo & YouTube videos are inserted on a web page using an iframe. This needs to be made responsive and the most efficient way in terms of code is to use a container with bottom padding applied as an intrinsic aspect ratio. If a widget for inserting these types of video does not have the means to adjust the ratio, any video with an aspect ratio other than 16:9 will be "letterboxed".
Video Aspect Ratio
Most Vimeo and YouTube videos conform to the standard HD Video aspect ratio of 16:9. However, older files may have an aspect ratio of 4:3 and professional movies will have a Cinemascope ratio of 21:9.
These video widgets calculate the correct aspect ratio from the data entered into the "Video Width" and "Video Height" boxes. These will accept either the video file dimensions or the numbers from the aspect ratio.
The examples of Vimeo and YouTube on this page insert a video with the Cinemascope ratio of 21:9 by entering "21" into the width box and "9" into the height box rather than the actual file dimensions.
YouTube
The YouTube video API has changed a lot recently and some of the older controls are redundant. For example, it is no longer possible to disable related videos.
It's possible to avoid all that junk at the end of a video by using a free Vimeo account instead of YouTube.