You are confusing backwards and forwards compatibility. Those sites may have added features that your iPad does not support, which is why it broke, if they have not added those, it might still work.

However JS is not 100% backwards compatible either, it is in many cases, largely backwards compatible, but there are rare cases of bug fixes, or deprecated APIs that might be removed and break old code, but this is not even JS itself, it's more like web/engine standards.