Paul I abolished Peter the Great's law that allowed each reigning emperor or empress to designate his or her successor and substituted a strict order of succession by proclaiming that the eldest son of the monarch would inherit the throne, followed by other dynasts according to primogeniture in the male line.
Later, Alexander I authorized the renunciation of succession rights of his brother and legal heir presumptive Constantine Pavlovich via a secret manifesto withheld until the Emperor's death.
In 1825, 1855, and 1881, concurrently with the accession manifests of the Emperors, their next living brothers were appointed regents in the eventual situation of their respective nephews (who were all then minors) succeeding to the throne before the legal majority.
In 1904, after the birth of Alexei, his uncle Michael was appointed the eventual regent in case Nicholas II died before his majority.
They held no provisions of the religion embraced by the monarch's consort or the spouses of those in the line of succession, unlike the Act of Settlement of England.
However, in practice, since 1740s, the brides of Romanov agnates, who were predominantly Protestant princesses of German states, typically did convert to Eastern Orthodoxy before the marriage and took a Russian name and patronymic that were later a part of their style as Grand Duchesses, such as Elizaveta Alekseevna, Anna Fedorovna, etc.
Exceptions were tolerated since 1870s for spouses of cadet Grand Dukes, not immediately in the line of succession, who might since then retain Protestantism but accepted a Russian style nevertheless.
There existed an idea, never enshrined formally in law, that a descendant of a mother who was not Orthodox had a dubious status as a successor.
Neither a morganatic wife of a reigning Emperor, in a precedent settled by Catherine Dolgorukova in 1880, had the style of an Empress (although Alexander II, which is far from certain, might consider crowning her later but failed to do so as he was assassinated soon thereafter).
Morganatic marriages of the Romanovs were not typically made public nor announced via manifestos, and if such spouses were created count(esse)s of prince(sse)s, their letters patent did not cite their marital status concerning the dynasty.
In the spring of 1911, Princess Tatiana Konstantinovna of Russia became engaged to Prince Konstantin Alexandrovich Bagration (1889–1915), a Georgian by birth who served in the Russian Imperial Guard regiment and would die in World War I.
On 30 November 1910, Grand Duke Constantine Konstantinovich noted in one of his posthumously-published journals (From the Diaries of Grand Duke Konstantin Konstantinovich, Moscow, February 1994) that he had received assurances that "they would never look upon her marriage to a Bagration as morganatic, because this House, like the House of Orléans, is descended from a once ruling dynasty."
The grand dukes then officially petitioned the Emperor by a commission chaired by Grand Duke Nicholas Nikolayevich of Russia, requesting that a new category of dynastic marriages be recognised, to consist of Imperial princes and princesses entitled, with specific Imperial consent, to marry persons of non-royal blood but retaining the right to transmit to the issue of eligibility to inherit the throne.
Also remaining unrepealed was Article 36 ("Children born of a marriage between a member of the Imperial Family and a person not of corresponding rank, that is, not belonging to a Royal or Ruling House, shall have no right of succession to the Throne").
The Emperor was present at the wedding and on the same day issued yet another ukase (#35766): "By Our and Grand Duke Konstantin Konstantinovich's and Grand Duchess Elizaveta Mavrikievna’s consent, the wedding took place on 24th day of this August [old style] of the Daughter of Their Imperial Highnesses, Her Highness the Princess Tatiana Konstantinovna, with Prince Konstantin Bagration-Mukhransky.