Grey lines with obscure formats (before playing)

[oas777]

There are grey lines around the video player with 4:3 videos. They disappear when you start the video. Cf. image attached and
https://video.ethz.ch/speakers/distinguished_speakers.html.

[turro]

In that webpage the player iframe is 400px height and 100% width. So depending on the actual width the player won't fit perfectly in the iframe and then it will show those lines