Probably not much. Fuel tanks are usually made of plastic now, so you aren't likely to create a spark and ignite something.

Petrol fuel in liquid form is hard to ignite, so if it leaks not much is going to happen other than giving your driveway a good clean (it's a solvent) - it's the gas form that is much more flammable. When petrol enters the engine combustion chamber it is mixed with air to form a flammable gas - there should be no liquid.

Diesel fuel is much harder to ignite, heck, you could probably use diesel fuel in place of engine oil and it probably wouldn't ignite.