Comment by pornel

Even with the best intentions, the implementation is going to have bugs and quirks that weren't meant to be the standard.

When there's no second implementation to compare against, then everything "works". The implementation becomes the spec.

This may seem wonderful at first, but in the long run it makes pages accidentally depend on the bugs, and the bugs become a part of the spec.

This is why Microsoft has a dozen different button styles, and sediment layers of control panels all the way back to 1990. Eventually every bug became a feature, and they can't touch old code, only pile up new stuff around it.

When you have multiple independent implementations, it's very unlikely that all of them will have the same exact bug. The spec is the subset that most implementations agree on, and that's much easier to maintain long term, plus you have a proof that the spec can be reimplemented.

Bug-compatibility very often exposes unintended implementation details, and makes it hard even for the same browser to optimize its own code in the future (e.g. if pages rely on order of items you had in some hashmap, now you can't change the hashmap, can't change the hash function, can't store items in a different data structure without at least maintaining the old hashmap at the same time).