Different browsers don't run code the same way, that's part of implementation, but the same browser should run the same code the same way. It's nearly the definition of computation to be predictable.

Similar to law, that's like leaving something purposely ambiguous and letting the courts sort it out later and get more specific. But with computers, that's nearly impossible and wouldn't produce usable software You can't just write some code intended to run in the browser, and release it and expect the browser to do something sane, reasonable, desired, or even logical after the fact. The testing for intended results needs to happen before the code is released and is "used".