Whenever I'm unsure of the preferred spelling for a particular word, I refer to
Google Trends to compare the search volumes for the different spellings and find the one that is used by the majority.
This helps remove any Australian / British English bias from my web pages and
optimize them for a global audience.
A recent comparison of
supersede gave a surprising result:
It would appear that the word
supercede seems to have been well and truly
superseded some time around 2012.
Ian Fieggen, 12-Mar-2016