HTTP headers are defined in RFC1945 (HTTP/1.0) and RFC2068 (HTTP/1.1). Note that RFC2068 states that multiple headers with the same name may be present only if the values may be concatenated.
HTTP headers may be generated by CGI scripts, and in Apache and CERN httpd by using a side file containing metadata. Other servers may have other mechanisms to generate headers. Note that certain server-generated headers may not be overridden (such as Date), and that others are only meaningful with a non-200 status code. Using an HTTP header is preferable to using META tags, since the header will be understood by cache agents and proxies in addition to browsers, and metadata (such as PICS data) may be associated with image files, sound files, etc.
However, new HTTP headers should not be created without checking for conflict with existing ones since it is possible to interfere with server and proxy operation.
The date and time after which the document should be considered expired. Controls cacheing in HTTP/1.0. In Netscape Navigator, a request for a document whose expires time has passed will generate a new network request (possibly with If-Modified-Since). An illegal Expires date, e.g. "0", is interpreted as "now". Setting Expires to 0 may thus be used to force a modification check at each visit.
Web robots may delete expired documents from a search engine, or schedule a revisit.
Dates must be given in RFC850 format, in GMT. E.g. (META tag):
<META HTTP-EQUIV="expires" CONTENT="Wed, 26 Feb 1997 08:21:57 GMT">or (HTTP header):
Expires: Wed, 26 Feb 1997 08:21:57 GMTIn HTTP 1.0, an invalid value (such as "0") may be used to mean "immediately".
See also CacheNow for discussion about cache control, page expiry, etc.
The HTTP content type may be extended to give the character set. As an HTTP/1.0 header, this unfortunately breaks older browsers. As a META tag, it causes Netscape Navigator to load the appropriate charset before displaying the page. E.g.
<META HTTP-EQUIV="Content-type" CONTENT="text/html; charset=ISO-2022-JP">
May be used to declare the natural language of the document. May be used by robots to categorize by language. The corresponding Accept-Language header (sent by a browser) causes a server to select an appropriate natural language document. E.g.
<META HTTP-EQUIV="Content-language" CONTENT="en-GB">or (HTTP header)
Content-language: en-GBlanguages are specified as the pair (language-dialect); here, English-British
Specifies a delay in seconds before the browser automatically reloads the document. Optionally, specifies an alternative URL to load. E.g.
<META HTTP-EQUIV="Refresh" CONTENT="3;URL=http://www.some.org/some.html">or (HTTP header)
Refresh: 3;URL=http://www.some.org/some.htmlIn Netscape Navigator, has the same effect as clicking "Reload"; i.e. issues an HTTP GET with Pragma: no-cache (and If-Modified-Since header if a cached copy exists).
Specifies the named window of the current page; can be used to stop a page appearing in a frame with many (not all) browsers. E.g.
<META HTTP-EQUIV="Window-target" CONTENT="_top">or (HTTP header)
Window-target: _top
Defines the name of an alternate cache to Netscape Navigator. E.g.
<META HTTP-EQUIV="Ext-cache" CONTENT="name=/some/path/index.db; instructions=User Instructions">
Platform-Independant Content rating Scheme. Typically used to declare a document's rating in terms of adult content (sex, violence, etc.) although the scheme is very flexible and may be used for other purposes.
See also the PICS HOWTO.
Specifies the action of cache agents. Possible values:
Specifies that alternates are available. E.g.
<META HTTP-EQUIV="Vary" CONTENT="Content-language">or (HTTP header)
Vary: Content-languageimplies that if a header Accept-Language is sent an alternate form may be selected.
Controls Web robots on a per-page basis. E.g.
<META NAME="ROBOTS" CONTENT="NOINDEX,FOLLOW">Robots may traverse this page but not index it.
A short, plain language description of the document. Used by search engines to describe your document. Particularly important if your document has very little text, is a frameset, or has extensive scripts at the top. E.g.
<META NAME="description" CONTENT="Citrus fruit wholesaler.">
Keywords used by search engines to index your document in addition to words from the title and document body. Typically used for synonyms and alternates of title words. E.g.
<META NAME="keywords" CONTENT="oranges, lemons, limes">
Typically the unqualified author's name.
Typically the name and version number of a publishing tool used to create the page. Could be used by tool vendors to assess market penetration.
Undefined.
Typically an unqualified copyright statement.
Simple content rating.
Based on an early version of the Dublin Core report, using a defined schema of document types such as FAQ, HOWTO.
Dublin Core Elements. See the Reference Description
HTdig tags. See the HTdig META page.
Deprecated:
Any other META tags in use ? Please let me know (andrew@vancouver-webpages.com).
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