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https://w3c.github.io/webdriver/webdriver-spec.html#finding-elements-in-a-document "Let value to a stating that the strategy is invalid." This appears twice in this section. I'm not certain what the exact wording should be.
https://github.com/w3c/webdriver/commit/cf0a657d48c97c2f3b1d02f3c4d67b0dc9849e5e