Right. While this is an open process, and browser vendors are well aware of these when the spec is announced, the only way to enforce this in a meaningful way w

Right. While this is an open process, and browser vendors are well aware of these when the spec is announced, the only way to enforce this in a meaningful way would be for all the vendors to coordinate... which I don't exactly see happening. There's no reason for vendors to delay a feature just because their competitors aren't ready yet. As for availability of these two features: stable Chrome supported Array.prototype.includes in Dec 2015 and in Jul 2016; stable Firefox supported these in Dec 2015 and Mar 2017, respectively. I guess you could say that these features were widely available in 2016, _just if you use Chrome_, but that's not a satisfying answer. MDN pages: [0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes#Browser_compatibility https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe... [1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Browser_compatibility https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe... Dates for corresponding browser versions drawn from the wikipedia pages: [2] https://en.wikipedia.org/wiki/Firefox_version_history https://en.wikipedia.org/wiki/Firefox_version_history [3] https://en.wikipedia.org/wiki/Google_Chrome_version_history https://en.wikipedia.org/wiki/Google_Chrome_version_history