Does HTML 5 Gaming use Canvas and Javascript? How to prevent cheating in this case?
For instance, what if a user uses GreaseMonkey or some tool to alter the script so that it just reports a perfect game score to the server every time?
I'll answer your question with a question of my own: Why don't we see rampant cheating in World of Warcraft? Or any other MMORPG for that matter?
The answer is because the game manufacturer keeps all game state serverside.
If I'm the client and I'm in charge of reporting damage done to the enemy, or my location, yeah of course I can cheat and tell the server I did 1 million damage.
On the other hand if the server keeps track of state and I only inform the server of changes to state, all I can do is tell the server "I moved left" or "I fired my gun" <-- anti-cheating is built into the architecture of the game
Of course, you did bring up the point of "perfect actions" - the case would be an action game, a shooter, etc. where objects could be identified and the "perfect action" send. And to that I'd say: this sort of cheating already exists, really. And it's existed for many years. So I don't see how anything changes just because these are javascript games.