how a Roman emperor crowned?

[FlaviusCrispus]

The idea of the Emperor being some kind of "super civilian magistrate" was always, from Augustus on, a "pleasant fiction." His power always rested on the legions, first and foremost, though many rulers had little or no military experience before ascending the throne (Caligula and Nero come to mind). Septimius Severus wasn't really any more of a "dominate" ruler than, say, Vespasian; in fact they had very similar careers and pathways to the throne (and they were each succeeded by their two sons). All emperors from Augustus to Macrinus were Senators first and had held some senior magistracy like the Consulship before becoming emperor, so this preserved the fiction of civilian supremacy. The age of the "Soldier Emperors" really commences with Maximinus I Thrax (AD 235-238), and even then proceeds in fits and starts until the later 3rd century, with the Great Anarchy (AD 238-268) and the advent of the Danubian clique under Claudius II Gothicus. Diocletian was a member of this cadre and formalized a lot of the traditions that had taken root since the onset of the Great Anarchy.