SVR3: Using HTTP referer to ensure that the only way to access non-conforming content is from conforming content

The objective of this technique is to ensure that users can obtain an accessible version of content where both non-conforming and conforming versions are provided.

Conformance Requirement 1 allows non-conforming pages to be included within the scope of conformance as long as they have a "conforming alternate version". It is not always possible for authors to include accessibility supported links to conforming content from within non-conforming content. Therefore, authors may need to rely on the use of Server Side Scripting technologies (ex. PHP, ASP, JSP) to ensure that the non-conforming version can only be reached from a conforming page.

This technique describes how to use information provided by the HTTP referer to ensure that non-conforming content can only be reached from a conforming page. The HTTP referer header is set by the user agent and contains the URI of the page (if any) which referred the user agent to the non-conforming page.

To implement this technique, an author identifies the URI for the conforming version of the content, for each non-conforming page. When a request for the non-conforming version of a page is received, the server compares the value of the HTTP referer header against the URI of the conforming version to determine whether the link to the non-conforming version came from the conforming version. The non-conforming version is only served if the HTTP referer matches the URI of the non-conforming version. Otherwise, the user is redirected to the conforming version of the content. Note that when comparing the URI in the HTTP referer header, non-relevant variations in the URI, such as in the query and target, should be taken into account.

Examples

Example 1: Interactive demonstrations of physical processes

An online physics course uses a proprietary modeling language to provide interactive demonstrations of physical processes. The user agent for the modeling language is not compatible with assistive technology. The site includes a script that uses the HTTP referer to ensure that unless users attempt to access the interactive demonstration from a page that contains a conforming description of the process and models, the server redirects the request to a conforming page which contains a link to the non-conforming version. Students may choose to access the non-conforming, interactive version, but those who do not are still able to learn about the process.

Example 2: Using Http referer in PHP

The following example illustrates how this technique can be used in PHP. It includes two files, conforming.php and non-conforming.php which work together to ensure that the only way to access non-conforming content is from conforming content.

conforming.php:

Example Code:


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
	<head>
    		<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
    		<title>Conforming Content</title>
    	</head>
	<body>
		<h1>This is a conforming page</h1>
		<p>From here, you can visit the <a href="non-conforming.php">non-conforming 
		page</a>. </p>
	</body>
</html>

non-conforming.php:

Example Code:


<?php 
// if the request comes from a file that contains the string "conforming.php" then render the page
	if(stristr($_SERVER['HTTP_REFERER'], "conforming.php")) {
?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
	<head>
		<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
		<title>Non-Conforming Content</title>
	</head>
	<body>
		<h1>This is a non-conforming page</h1>
		<p>Because you came from <?php echo $_SERVER['HTTP_REFERER']; ?>, you are 
			able to view the content on this page. </p>
	</body>
</html>
<?php
}
// if the referring page is not conforming.php, then redirect the user to the conforming version
else  {
header("Location: conforming.php");
}
?>

Working Example

Related Techniques

Tests

Procedure

Where WCAG conforming alternatives are provided for non-conforming content:

Identify pages that do not conform to WCAG at the conformance Level claimed where accessible alternatives are served based on HTTP Referrer.
Visit the URI of the non-conforming content.
Verify that the resulting page is one of the following:
1. a conforming alternate version for the non-conforming content
2. a page that includes a link to both the conforming alternate version and the non-conforming content

Expected Results

Check #3.1 or #3.2 is true.