Comment by paxcoder

Undefined behavior has a purpose: Not specifying implementation details makes it easier to write new implementations and for a wider variety of platforms. "De facto standards" take away this freedom, so ideally you'd want to reject reliance on UD, but I see your (second) point about that not always being practical. I guess "be conservative in what you do, be liberal in what you accept from others". Just make sure that your foundations are strong (pun) or the whole house will be an EcmaScript.