he emperor had to be male and at least 18 years old, he had to live within the Empire, and was expected to have four quarters of nobility (i.e. all of his four grandparents had to be noble).
No law mandated that he should be catholic: although many texts took this condition for granted, many lawyers never saw it as a fundamental prerequisite.
Neither did he have to be German, and if fact Alfonso of Castille and Charles V weren’t.
Since the XVII century it became nevertheless customary that the candidate should possess estates within the Empire.
In 1648 the king of France ceded Alsace to the Empire so that he may have the chance to be elected the Kaiser.
The title of emperor was not hereditary, but elective. However between 1453 and 1740, with brief interruptions, the emperor was always a Habspurg.
He could propose, approve, promulgate as well as veto laws, but he could not increase taxation without approval.
He was the supreme judge, but could only exercise this prerogative in the courts of justice. He did not have the right of speech in the Reichskammergericht, even though in some cases he did have the last word in the Reichshofrat.
He could grant pardons, bestow exemptions and privileges.
He represented the Empire, but the right to sign treaties was quite soon curtailed.
He was at the top of the feudal pyramid.