http-equiv (HTML attribute)

Description

You can apply many settings to a web server in the form of HTTP response headers—information that’s sent back with the response to any request made for a resource on that server. However, not everyone has direct access to their web site’s server configuration options; if this is the case, you can use the meta element to simulate an HTTP response header. For example, the character encoding may be set at the server, but many web authoring packages include character encoding as a meta element just in case this important information is not sent by the server.

Typical uses for the http-equiv attribute include managing cache control, page refreshes, and page content safety ratings. If the http-equiv attribute is used in the meta element, the name attribute shouldn’t be used.

Note that supplying an HTTP equivalent in a meta element is only effective if the server doesn’t send the corresponding real header; you can’t override an HTTP header with a meta element. So using this attribute is only of value when the server doesn’t send that particular header, or when there is no server involved—for instance, when you’re viewing a document from the local file system.

Value

The value of http-equiv varies depending on the way you use it. It may contain any of the following values:

• "Allow"
• "Content-Encoding"
• "Content-Language"
• "Content-Length"
• "Content-Type"
• "Date"
• "Expires"
• "Last-Modified"
• "Location"
• "Refresh"
• "Set-Cookie"
• "WWW-Authenticate"

Note that "Refresh" is a nonstandard HTTP header extension, the use of which is strongly advised against for accessibility reasons: it takes the control of the page away from the user. This is particularly annoying if the user is having to access the page using a screen reader and the content isn’t read out completely before the page refresh kicks in. Using this type of meta attribute value will cause a failure against checkpoints defined in the Web Content Accessibility Guidelines versions 1.0 and 2.0 (still in draft format at time of writing).

Compatibility

While http-equiv is designed for containing HTTP header information, it doesn’t have many applications except with the values of "Content-Type" and "X-UA-Compatible" (introduced in IE8 for handling standards compatibility; a good explanation of this issue can be found in the A List Apart article, “Beyond DOCTYPE: Web Standards, Forward Compatibility, and IE8”).