In Austria, there was an official list of acceptable houses for intermarriage with the imperial dynasty. It was known as the Habsburg List of 1900, official but non-published until the controversy surrounding the marriage of Archduke Franz Ferdinand and the Bohemian Countess Sophie Chotek. According to it, his wife was of insufficient rank for a dynastic union, insofar as she did not belong to any of the listed houses (in contrast to Princess Isabella of Croÿ, who belonged to a mediatized house, and hence had been able to marry Archduke Karl, Duke of Teschen). As such, Emperor Franz Joseph metaphorically had his hands tied: he was in no position to grant consent to the match. Hence, the result was a morganatic marriage (arguably the most famous in history).
In Russia, however, things were different: in the first place, Ebenbürtigkeit wasn't even officially in the original code of the Fundamental Laws; rather, it got added only later on (perhaps to retroactively justify the accession of Czar Nicholas I to the imperial throne, instead of his older brother Constantine). As such, it wasn't defined by an official list of acceptable houses for intermarriage with the Romanovs.
Rather, *equality of birth* was defined to be dynastic membership in either a ROYAL or REIGNING house. As such, Princess Hélène of Orléans (born in 1871), daughter of the Count of Paris (Henri, pretender to the defunct throne of France), despite being a member of a deposed dynasty, was at one point in serious contention for becoming the bride of the future Czar Nicholas II.
As it was, religion got in the way of the match (as in the earlier case of the potential match between the French princess and the Duke of Clarence). So did the czarevich's infatuation with Princess Alix of Hesse-Darmstadt, who overcame her religious scruples and objections to eventually convert from Lutheranism to Orthodoxy. Unlike Hélène, she did not come from a royal house; but the grand duchy she came from was nonetheless still REIGNING. Hence, the "equality" of the marriage that eventually took place (1894).
So perhaps the Romanovs actually upheld higher marital standards than the Habsburgs, after all, insofar as mediatized houses would not have been acceptable for intermarriage with members of the Russian imperial dynasty. As it was, the Austrian imperial dynasty accepted not only Croÿ but also Thurn-and-Taxis (Albert, the 8th prince, married Archduchess Margarete Clementina of Austria-Hungary).
However, there was a catch: earlier in European royal history, King Maximilian II of Bavaria initially hesitated about granting consent to the marriage of Duchess Helene in Bavaria to Albert's father, Hereditary Prince Maximilian of Thurn-and-Taxis. Because the house was not a reigning dynasty, he considered the union a mésalliance: the way he saw it, his kinswoman was marrying beneath herself. As it was, it took the intervention of her sister, Empress Elisabeth of Austria, who interceded with Emperor Franz Joseph on behalf of her older sister, for him to relent in his opposition and allow the marriage to take place.
Perhaps the imperial couple acted in part out of guilt because earlier on, the emperor had been expected to make Helene into his bride. Instead, he got swept off his feet by her younger sister. So perhaps it was an act of *compensation* on the part of Franz Joseph, to write to the Bavarian king to put in a good word for Helene's new suitor, to set aside objections to his *inferior* rank.
Well, with the genealogical tie established, he certainly couldn't say no later on to the marriage of Margarete to Helene's son, who was his nephew by marriage (and also of King Francesco II of the Two Sicilies).
Anyhow, getting back to the subject of royal Bavaria, I was just wondering what exactly the Wittelsbach house laws stated as constituting *equality* of birth. Certainly Luitpold, the prince-regent of the kingdom, couldn't withhold consent to the marriage of his grandson Franz to the younger Princess Isabella of Croÿ (niece and namesake of the archduchess who gave Franz Ferdinand and Sophie a hard time), since her aunt had been able to marry into imperial Austria (back in 1878).
I know that there was another mediatized house which was accepted for a dynastic intermarriage with the Wittelsbachs, between the 1858 union with Thurn-and-Taxis and the 1912 union with Croÿ: the 1898 marriage of Helene's niece Sophie, Duchess in Bavaria and Count Hans Veit of Törring-Jettenbach. Perhaps the prince-regent granted consent to the match as a reward to the groom's father, Count Clemens, for helping him depose his nephew, the famous "Mad" King Ludwig II ...