With some providers (Youtube from what I could find) we have aspect ratio problems when we render them using the OembedFormatter.
The problem I discovered is primarily with Youtube: the resource returned from Youtube does not contain the requested width/height but the default size (or simply A size) of the video. This is fine if the formatter is configured to use a width and height that corresponds to the aspect ratio of that particular video. But that won't ever happen. When we configure the formatter to a specific max width but a height of 0, there is at least some expectation that the rendered iframe maintains the aspect ratio. At the moment, since the formatter is configured to a *max* width and no *max* height the iframe gets that that width attribute and the default height that comes from the resource. So the result is that the iframe size doesn't match the size of the video. A solution for this is to calculate the aspect ratio of the video using the values in the resource and recalculate the height attributed based on that in case the max height is configured as 0.
Discussing with @phenaproxima we might make a checkbox (default unchecked) for this in order to prevent BC.
Other providers, like Vimeo, may not have this issue. So in these cases this should not apply. But youtube is pretty important.