The first time I had a group of players face Strahd, it was c. 1988 and we were playing 1e with the original I6 Ravenloft. The PC's quickly found their way to the lowest level of the castle, eventually found the crypts and destroyed Strahd's coffin, and then spent hours with Strahd sniping at them and regenerating before they were finally able to corner him. Reduced to 0hp and without a coffin to retreat to, I ruled that he had been destroyed.
The next time was in 2016 in a 5e game, but just before the release of Curse of Strahd. I used the original I6 and an online fan conversion for 5e. That time, the PCs ventured to the crypts but quickly retreated after a number of them were charmed by Strahd. They later holed up in a room that Strahd could only enter or leave in mist form, and when he entered they cast moonbeam to force him out of mist form and daylight to damage him until he died. Given the description of vampiric mists in Mordenkainen's Tome of Foes, I ruled that at 0hp, unable to regenerate and unable to return to his intact coffin, Strahd was destroyed.
My most recent game just concluded, running Curse of Strahd. This played out very similarly to the first time, except that the players found his coffin even more quickly (it really is ridiculously easy to find, if you think about a sensible layout for a castle). They destroyed the coffin by pouring most of the holy water that Ezmerelda carried into the grave dirt within it. A similar sniping game as the first time ensued, with Strahd removing one of the party's Fortunes and other helpful items each time he attacked, until he grew too bold and was reduced to 0hp without a coffin to return to. Again, I ruled that he had been destroyed.
It was only after I had wrapped up the game that I realized the book says (emphasis mine):
When Strahd is reduced to 0 hit points, he turns to mist and retreats to his coffin (see the Misty Escape feature in his stat block). The vampire must be in his resting place to be utterly destroyed.
If the characters finish off Strahd in his coffin, read:
I would like to run Curse again, but am confused about how to proceed. Judging by the text of the module, what should have happened to Strahd if his coffin was destroyed and then he was reduced to 0hp? Or, does the module assume that it is not possible to destroy the coffin? What should happen to Strahd if his coffin is intact but he is unable to return to it at 0hp?