Greasy Fork is available in English.
Middleclick a YT video to crop black borders and fill the viewport (without loosing content)
Middle-click a YouTube video to crop all encoded black borders and fill the viewport as good as possible (keeps aspect ratio and looses no content).
Keep in mind that it only analyzes the current frame for black borders and remembers the crop&zoom! So make sure you don't enable it in a dark scene.
Otherwise, just update the crop&zoom by disabling and reenabling the feature (middle-clicking twice).
Should work on also on embedded YT videos. YT red is untested!