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"fulfil" I believe fulfill is more common, though fulfil is technically okay.
Fulfil is the UK spelling of the word and might have slipped through since most of us live in the UK (-: https://github.com/w3c/webdriver/pull/158
landed in https://github.com/w3c/webdriver/commit/c7d82d84aae5fa904acf09b1950772312b711d8c