The problem is that it masks the error. Then you have all kinds of websites that only work in some browsers, or have an API that is not accessible from some aut

The problem is that it masks the error. Then you have all kinds of websites that only work in some browsers, or have an API that is not accessible from some automated tools unless you enable "insecure mode", and everybody is worse off. It also makes implementing new browsers more complicated, and thus the ecosystem less open. All that to hide a configuration error that would take a second to fix in the first place.