Driving LED with FPGA
It would be a very bad idea to do this in a product where reliability was of any importance, particularly if more than a few I/Os were being abused in this way. Even if the actual current is not high enough to shorten the life of the LED you may shorten the life of FPGA

Read up on electromigration failures, and read the fine print on maximum current per I/O bank GND/Vcc and such like. 

Tolerances for things like current (MOSFET Idss in this case) can be very broad in semiconductor processing. An "8mA" (for the purposes of briefly charging/discharging stray capacitance) might be 16mA or 24mA. There is not a precision current source/sink in there, it's just a MOSFET.