JS used to not require that iteration order match definition order. Iteration order was explicitly unspecified. But initial implementations just stored proper

JS used to not require that iteration order match definition order. Iteration order was explicitly unspecified. But initial implementations just stored properties in a flat list, so in practice all had iteration order matching definition order. And then websites were created depending on that behavior. So now you had a spec saying "the order can be anything" but real-world compat constraints meant that in any implementation shipping in a web browser the order had to be definition order. At some point this compat constraint was simply documented in the spec, so there wouldn't be a gotcha lurking for a new implementation. So it's hard to blame this problem on JS, really. It's more like Postel's Law (or a failure thereof, depending on who you're talking to, but I feel like in this case the output is the iteration order, and having it be fixed corresponds to being conservative in what you do. The pages that failed on different iteration orders correspond to _not_ being liberal in what you accept from others...