Meta elements of the form <meta http-equiv="foo" content="bar"> can be used as alternatives to HTTP headers. For example, <meta http-equiv="expires" content="Wed, 21 June 2006 14:25:27 GMT"> would tell the browser that the page "expires" on June 21, 2006 at 14:25:27 GMT and that it may safely cache the page until then. The HTML 4.01 specification optionally allows this tag to be parsed by HTTP servers and set as part of the HTTP response headers,[26] but no web servers currently implement this behavior.[27] Instead, the user agent emulates the behavior for some HTTP headers as if they had been sent in the response header itself.