As for the germane Empire after 1871 there where no Princes and Princesses of Germany. Only the Crown Prince and Crown Princness had the Title of the german Empire and of Prussia and had the Stlye of Imperial and Royal Highness which is now also the stlye of the Head of the royal house of Prussuia. But Prince Georg Friedrich only uses Royal Highness uznlike his grandfahter who used Impoerial and Royal Highness.
Were only the Habsbugs styled thus? Did it have to do with the fact that they were archdukes and archduchesses of Austria, but (say) princes and princesses of Tuscany (a grand duchy, until deposed in 1860) -- or other cadet branches of the house (e.g. Modena, Teschen)?
Of course, it has been stressed that only members of the imperial line of the Habsburgs (i.e. male-line descendants of Holy Roman Emperor Francis II) were Imperial Highnesses, the others being *only* Royal Highnesses. In fact, the Habsburgs supposedly didn't acquire imperial rank until 1804, when the Holy Roman empire got dissolved and the Austro-Hungarian empire was formed (when Francis II became Emperor Francis I). But of course, many sources (e.g. Wikipedia) continue referring to members of the other branches also as Imperial Highnesses ...
What about princes of the German empire? Were they also styled as Imperial and Royal Highness? It seems logical that they would have, since they would have been Imperial Highnesses as princes of Germany, but Royal Highnesses as princes of Prussia.
In Russia, however, there was no such thing as a Royal Highness: the highest qualifying rank of a cadet member of the Romanov dynasty was Imperial Highness, styled by grand dukes and grand duchesses. If one was not a child or grandchild through son of a sovereign, but only dynastic male-line grandchild, he was a Prince(ss), with the qualification of Highness. Beyond that, one was a Prince(ss), with the qualification of Serene Highness.
Matters were further complicated whenever a princess left her native land to marry a foreign prince (which has traditionally been expected to happen). If the union was dynastic, and she married *up*, then no problem: her lower qualifier (e.g. Grand Ducal Highness) would get dropped altogether, and the higher one (e.g. Royal Highness) would get adopted.
But if she married *down*, one ran into potential complications: a Romanov grand duchess who was an Imperial Highness might marry a Royal Highness who was a cadet member of a kingdom (e.g. the Duke of Edinburgh, second son of Queen Victoria of Great Britain), or a sovereign grand duke in Germany (e.g. Mecklenbuerg, Baden). As I understand, the Duchess of Edinburgh was doubly styled as Imperial and Royal Highness: it's just that her mother-in-law insisted on the Royal being first in the UK, the Imperial second. The strong-willed daughter-in-law, however, resisted -- and evidently got her way on the matter.
If, however, an Imperial Highness married only a Grand Ducal Highness or Highness, there should have been no double styling. Rather, she would simply have remained an Imperial Highness -- correct?
In the case of Archduchess Gisela of Austria (daughter of Emperor Franz Joseph), the qualifier of Royal Highness she acquired at marriage (in 1873, to Prince Leopold of Bavaria) would have been redundant. After all, she was born an Imperial and Royal Highness (unlike the Grand Duchess Maria Alexandrovna of Russia, who was simply HIH). So I assume that nothing changed, as the Bavarian royal court permitted her to continue styling herself Imperial first, Royal second -- correct?