When the http-equiv attribute is specified on a meta element, the element is a pragma directive. You can use this element to simulate an HTTP response header, but only if the server doesn't send the corresponding real header; you can't override an HTTP header with a meta http-equiv element.
从 W3C 的对 http-equiv 如上定义来看,如下的覆盖的说法不妥
标签的http-equiv属性用来覆盖 HTTP 回应的头信息字段