Examples of how to embed a responsive video iframe with loading deferred until it's in the viewport.
Use Bootstrap's container, row and column layout classes to control sizing.
Use Bootstrap's ratio layout classes to control aspect-ratio of video container.
16:9 Youtube
16:9 Vimeo
9:16 demo
Also includes a ratio-9x16--limit-to-vh class to limit the height so the video is no deeper than the viewport height minus 30px.