From the http spec about 4xx errors: > Except when responding to a HEAD request, the server SHOULD send a representation containing an explanation of the error

From the http spec about 4xx errors: > Except when responding to a HEAD request, the server SHOULD send a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition. So sending content alongside a 4xx error should be the standard behavior and browsers should display that content. Not sending/displaying that content is the non-standard behavior.