The keyboard shortcuts that you can use while watching Prime Video on a web browser are:

SPACE to toggle play/pause.

F to enter or exit full screen.

Esc to exit full screen or exit playback.

Left arrow to rewind 10 seconds.

Right arrow to fast-forward 10 seconds.

Up arrow to increase volume.

Down arrow to decrease volume.

M to toggle mute.

C to toggle subtitles/captions on and off, and to switch between available subtitle/caption languages.

A to toggle through available audio tracks, including audio descriptions and Dialogue Boost.