Remote videos (i.e., from YouTube or Vimeo) are displayed inside an iframe for security reasons. Within the iframe, the videos themselves are also served as iframes. Before #2998091: Remote videos overflow their containing element, neither the outer iframe nor the inner iframe were responsive -- they could and would overflow their containing element, which could break responsive layouts.
Now, the media system attaches CSS libraries to both frames to make them responsive, so that they no longer overflow. This is accomplished by two new asset libraries: media/oembed.formatter, which is attached to the outer frame, and media/oembed.frame, which is attached to the inner frame.
These libraries only contain CSS, and it is not yet possible to add JavaScript to the media/oembed.frame (inner frame) library (this is being discussed in #3042383: Document how to attachment JavaScript inside the oEmbed iframe).
If you want to add your own CSS to either frame, you can extend, override or remove one or both of the libraries in your theme. If you want to revert this change completely in your theme, the following addition to the theme info.yml file will keep your theme intact from the change:
libraries-override:
media/oembed.formatter: false
media/oembed.frame: false
