I know that Aachen was the chief city in the days of Charlemagne, and that Nuremberg has been important at various stages and had an official residence, but when did Vienna become the der facto capital of the HRE? Thanks!
There was no official capital, the governing body of the Holy Roman Empire, the Imperial Diet, met in many places. Of supreme importance were the cities of Frankfurt, Nuremberg, Regensburg and Augsburg. Vienna was never a capital of the HRE.