Using <meta> Tags (Web Design in a Nutshell, 2nd Edition)

9.4.1. The http - equiv Attribute

Information provided by an http-equiv attribute is processed as though it had come from an HTTP response header. HTTP headers contain information the server passes to the browser just before it sends the HTML document. It contains MIME type information and other values that affect the action of the browser. Therefore, the http-equiv attribute provides information that will, depending on the tag description, affect the way the browser handles your document.

There are a large number of predefined http-equiv types available. This section will look at just a few of the most useful. For a complete listing, see the Dictionary of HTML META Tags at http://vancouver-webpages.com/META/.

9.4.1.1. Meta tags for client-pull

Client-pull refers to the ability of the browser (the client) to automatically request (pull) a new document from the server. The effect for the user is that the page displays, and after a period of time, automatically refreshes with new information or is replaced by an entirely new page. This technique can be used to automatically redirect readers to a new URL (for instance, if an old URL has been retired). Be aware, however, that the W3C strongly discourages the use of this method for automatic forwarding in favor of server-side redirects.

If you string documents with client-pull instructions and set very short time intervals, you can create a sort of slide show effect. Client-pull uses the refresh attribute value, first introduced by Netscape. It tells the browser to wait a specified number of seconds (indicated by an integer in the content attribute) and then load a new page. If no page is specified, the browser just reloads the current page. The following example instructs the browser to reload the page after 15 seconds (we can assume there's something fancy happening on the server side that puts updated information in the HTML document):

<META HTTP-EQUIV="refresh" CONTENT="15">

To reload a different file, provide the URL for the document within the content attribute as shown below:

<META HTTP-EQUIV="refresh" CONTENT="1; URL=http://doc2.html">

Note that there is only a single set of quotation marks around the value for content. Although URLs usually require their own quotation marks, these are omitted within the context of the content attribute.

To create a slide-show effect, add a meta refresh tag in the <head> of each document that points to the next HTML document in the sequence. You can set the time interval to as many seconds as you like; setting it to 0 will trigger the next page as soon as the current page has downloaded. Bear in mind, however, that the actual amount of time the page takes to refresh is dependent on complex factors of file size, server speed, and general web traffic.

In the following example, three files are coded to loop endlessly at five-second intervals.

Document 1.html contains:

<META HTTP-EQUIV="refresh" CONTENT="5; URL=2.html">

Document 2.html contains:

<META HTTP-EQUIV="refresh" CONTENT="5; URL=3.html">

Document 3.html contains a tag which points back to 1.html :

<META HTTP-EQUIV="refresh" CONTENT="5; URL=1.html">

9.4.1.2. Other uses

Here are some other uses of the http-equiv attribute:

expires

content-type

The content-type text/html is automatically added to the HTTP header for HTML documents. This attribute can be extended to include the character set for the document by specifying "text/html; charset=character set identifier". This causes the browser to load the appropriate character set before displaying the page.

This is part of the HTML 4.01 measures to internationalize the Web. You can read more about identifying character sets in Chapter 7, "Internationalization".

<META HTTP-EQUIV="content-type" CONTENT="text/html; 
    charset=SHIFT_JIS">

content-language

This may be used to identify the language in which the document is written. Like the character set extension just mentioned, it is part of the ongoing effort to internationalize the Web. The browser can send a corresponding Accept-Language header, which causes the server to choose the document with the appropriate language specified in its <meta> tag.

For more information on internationalization and a listing of two-letter language codes, see Chapter 7, "Internationalization".

This example tells the browser that the document's natural language is French:

<META HTTP-EQUIV="content-language" CONTENT="fr">

9.4.2. Inserting Meta-Information with the name Attribute

The name attribute is used to insert hidden information about the document that does not correspond to HTTP headers. For example:

<META NAME="author" CONTENT="Jennifer Niederst">
<META NAME="copyright" CONTENT="2001, O'Reilly & Associates">

You can make up your own <meta> names or use one of the names put forth by search engine and browser companies for standardized use. A few of the accepted and more useful <meta> names are discussed in the following sections.