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What's currently "This must return true if the capabilities contain a key with the given capabilityName and the value of that key is defined. If the value is a boolean, this function must return that boolean value. If the value is null, an empty string or a 0 then this method must return false." could be made slightly simpler and shorter, for example, "This must return true if the capabilities contain a key with the given capabilityName and the value of that key is either the boolean True or a non-empty DOMString or a long that is not 0. Otherwise it must return false."
https://dvcs.w3.org/hg/webdriver/rev/8d2f8fc20402