Its a great idea, the only thing is I feel like the "FLIP" itself doesn't seem like a true flip. If you could do something else withe the UI to simulate a flip. We associate coin flipping with a coin going up in the air, the hand over it where you don't know the answer and then the reveal. Your site gets the job done, but kind of takes away the suspense.

Bookmarklet:

javascript:(function(){ const result = Math.random() < 0.5 ? 'Heads' : 'Tails'; alert('Coin flip result: ' + result); })();