Many desktop software (VLC, MPC) and have an option to 'zoom in' , 'crop black bars', or crop to a specific aspect ratio. How can we do this in Fullscreen on Youtube or other flash Video sites?

I am the viewer, NOT the video's creator/publisher.

iOS Can do this (double tap to zoom, which removes black bars, zoom depth not configurable).

afaik, Desktop computers (and android devices), cannot do this on the fly. The only 'workaround' I've found is F11 and zooming of the entire web page - basically a fake full screen and zooming the web page beyond the screen size.

Use Case: watching 4:3 flash videos from the web on a widescreen monitor.

Looking for all creative solutions, including any software that accesses YouTube without using a web browser.