Window tinting is addressed in sections 73 and 74 of the (Highway Traffic Act) whereby the surface of the windshield or any side window to the left or right of the driver cannot be coated with any colour spray or other colour coating in such a manner as to obstruct the driver's view of the roadway, or obscure the view from outside to the interior of the motor vehicle.
You have to be able to see outside, and other people, especially police, have to be able to see you.
Ontario's law doesn't specify a percentage of tint so it's up to police to decide whether your windows are too dark. If they decide they can't see you, you could be fined.
In British Columbia, Alberta, Saskatchewan and Nova Scotia, it's illegal to have any tint on driver and passenger side windows.
Generally there are no restrictions on tinting rear windows.