It depends on the output impedance of the IC in question axro. Somewhere in the datasheet it should tell you the conditions under which you can get 20ma out or otherwise specifically state the output impedance, regardless of the device you're going to need a current limit resistor, an LED will try to draw more than the 20ma that the IC is rated for and could damage the I/O line by trying to draw more than it's rated for. In general it's a good bet to assume the output impedance of any cmos based device is very low even if it can't provide a lot of current. The current limit is the physical limitation of the size of the output drivers.