Don't try this on a working WebBrick !!

 It will lead to thermal shutdown and possibly permanent damage to the digital output.

We made a test of a WebBrick DO to see if it could be destroyed in a short condition.  The specification of the processor claims that there is a current clamp at 20mA

Our graph shows that this is closer to 31mA. 

 Note that TTL '1' level is maintained to 20mA which is the declared rating.