A Dictionary of HTML META Tags
META tags have two possible attributes:
<META HTTP-EQUIV="name" CONTENT="content">
<META NAME="name" CONTENT="content">
META tags should be placed in the head of the HTML document,
between the <HEAD> and </HEAD> tags (especially important in
documents using FRAMES).
HTTP-EQUIV tags
META tags with an HTTP-EQUIV attribute are equivalent to HTTP
headers. Typically, they control the action of browsers, and may
be used to refine the information provided by the actual
headers. Tags using this form should have an equivalent effect
when specified as an HTTP header, and in some servers may be
translated to actual HTTP headers automatically or by a
pre-processing tool.
Note: While HTTP-EQUIV META tag appears to work properly with
Netscape Navigator, other browsers may ignore them, and they are
ignored by Web proxies, which are becoming more widespread. Use
of the equivalent HTTP header, as supported by e.g. Apache
server, is more reliable and is recommended wherever possible.
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.
Content-Disposition
Source: RFC2183 - Specify application handler (Microsoft), e.g.
Content-Type: text/comma-separated-values
Content-Disposition: inline; filename=openinexcel.csv
Expires
Source: HTTP/1.1 (RFC2068)
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 GMT
In HTTP 1.0, an invalid value (such as "0") may be used to mean
"immediately".
Note: While the Expires HTML META tag appears to work properly
with Netscape Navigator, other browsers may ignore it, and it is
ignored by Web proxies. Use of the equivalent HTTP header, as
supported by e.g. Apache, is more reliable.
See also CacheNow for discussion about cache control, page
expiry, etc.
Pragma
Controls cacheing in HTTP/1.0. Value must be "no-cache". Issued
by browsers during a Reload request, and in a document prevents
Netscape Navigator cacheing a page locally.
Content-Type
Source: HTTP/1.0 (RFC1945)
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">
It is now recommended to always use this tag, even with the
previously-default charset ISO-8859-1. Failure to do so may
cause display problems where, for instance, the document uses
UTF-8 punctuation characters but is displayed in ISO or ASCII
charsets.
Content-Script-Type
E.g.
<META HTTP-EQUIV="Content-Script-Type" CONTENT="text/javascript">
Source: HTML 4.0
Specifies the default scripting language in a document. See
MIMETYPES for applicable values.
Content-Style-Type
E.g.
<META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css">
Source: HTML 4.0
Specifies the default style sheet language for a document.
Default-Style
Source: HTML 4.0
Set the document's preferred style sheet, taken from an
stylesheet specified elsewehere e.g. in a LINK element.
Content-Language
Source: HTTP/1.0, RFC1766
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-GB
languages are specified as the pair (language-dialect); here,
English-British
Refresh
Source: Netscape
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.html
In 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).
Note: If a script is executed which reloads the current
document, the action of the Refresh tag may be undefined. (e.g.
<body onLoad= "document.location='otherdoc.doc'>)
Window-target
Source: Jahn Rentmeister
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
Ext-cache
Source: Netscape
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">
Set-Cookie
Source: Netscape Navigator
Sets a "cookie" in Netscape Navigator. Values with an expiry
date are considered "permanent" and will be saved to disk on
exit. E.g.
<META HTTP-EQUIV="Set-Cookie"
CONTENT="cookievalue=xxx;expires=Friday, 31-Dec-99 23:59:59 GMT;
path=/">
PICS-Label
Source: PICS
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. For PICS for Medical data, see
medpics.org.
Cache-Control
Source: HTTP/1.1
Specifies the action of cache agents. Possible values:
Public - may be cached in public shared caches
Private - may only be cached in private cache
no-cache - may not be cached
no-store - may be cached but not archived
Note that browser action is undefined using these headers as
META tags.
Vary
Source: HTTP/1.1
Specifies that alternates are available. E.g.
<META HTTP-EQUIV="Vary" CONTENT="Content-language">
or (HTTP header)
Vary: Content-language
implies that if a header Accept-Language is sent an alternate
form may be selected.
Lotus
The Lotus publishing tool generates Bulletin-Date and
Bulletin-Text attributes. Bulletin-Text contains a document
description.
NAME attributes
META tags with a name attribute are used for other types which
do not correspond to HTTP headers. Sometimes the distinction is
blurred; some agents may interpret tags such as "keywords"
declared as either "name" or as "http-equiv".
Robots
Source: Spidering
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.
Altavista supports:
NOINDEX prevents anything on the page from being indexed.
NOFOLLOW prevents the crawler from following the links on the
page and indexing the linked pages.
NOIMAGEINDEX prevents the images on the page from being indexed
but the text on the page can still be indexed.
NOIMAGECLICK prevents the use of links directly to the images,
instead there will only be a link to the page.
Google supports a NOARCHIVE extension to this scheme to request
the Google search engine from caching pages; see the Google FAQ
See also the /robots.txtexclusion method.
Description
Source: Spidering, AltaVista, Infoseek.
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
Source: AltaVista, Infoseek.
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">
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