I watch a lot of my media on my PC using Plex/Web. I would really love the same keyboard functionality as is found in VLC for example to be built into the Plex/Web player while the browser window has focus.
In particular this would be:
Press the space bar to pause / resume the video
Use the left and right cursor keys to rewind / fast forward
And (though it's not standard), use the up and down cursor keys for volume control.