After additional googling I found this:
"With JavaScript you can only detect Shift. You can't differentiate left and right shift.
The browser does not differentiate from left and right shift."
Well, it's pretty bad for fighting or football games (not many people like to play on a single keyboard, but I know that I should make this feature).
So, is there any other solution exept of writing a small program which starts with my game and defines left shift as f7 (for example) ?