All of these crypto functions have many implementations in every popular programming language that return the exact same bits no matter which implementation you use. As long as you have the keys (which is a serious issue) and someone remembers which function was used with which parameters the risk of "will be left with a bunch of HTML files with data that the browsers cannot decrypt anymore." is not even on the same scale as getting hit by lightning but rather on the same scale as getting hit by lightning during clear weather in a deep basement.
I remember a few years ago on Discovery Channel, there was a documentary about people hit by lightning. One was hit by lightning 3 times, all 3 times on forehead, and survived to tell the story on record (you could see his forehead having very clear view of its underlying sanguine vessels as secondary effect).
But one story that particularly stood out was of a woman who was hit by lightning during clear weather, inside the house, through the water pipes - she was cleaning dishes at the time. So your scale might be still too small.
