Techniques for WCAG 2.0

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SCR38: Creating a conforming alternate version for a web page designed with progressive enhancement

Important Information about Techniques

See Understanding Techniques for WCAG Success Criteria for important information about the usage of these informative techniques and how they relate to the normative WCAG 2.0 success criteria. The Applicability section explains the scope of the technique, and the presence of techniques for a specific technology does not imply that the technology can be used in all situations to create content that meets WCAG 2.0.


HTML with scripting.

This technique relates to:


This objective of this technique is to offer a conforming alternate version for a web page designed with progressive enhancement. The technique demonstrates how to use a scripting technique to accomplish this by:

To be accessible, these dialogs must follow a few simple rules.

  1. Storing the initial pre-enhanced version of the web page so that it can act as a "conforming alternate version" for any later enhanced versions of the content; and

  2. Inserting a mechanism into all enhanced versions of the web page which allows a user to revert the content back to the stored pre-enhanced Alternate Version.

Web pages designed with progressive enhancement detect features in the web-enabled accessing device (size, capability and software) to allow those supported web technologies to be applied in layers on top of an HTML foundation. The basic content and functionality of such a web page are available through the HTML foundation to anyone using a more simple web-enabled accessing device, whilst enhanced versions of the page are created to suit the different features in more advanced accessing devices.

The current guidance for web pages delivered in alternate versions reads: "Note 4: Alternate versions may be provided to accommodate different technology environments or user groups. Each version should be as conformant as possible. One version would need to be fully conformant in order to meet conformance requirement 1." With regard to web pages designed with progressive enhancement this leaves the problem of which version to select as the one fully conformant version - all whilst trying to ensure that no set of users is disadvantaged by that choice.

One solution to this challenge is to select the pre-enhanced version of the web page (e.g. the DOM state created solely from the HTML in the source code in the absence of support for scripts, styles or non-HTML plugins) as the "fully conformant version", due to its broad reach, with regard to support, across all the possible web-enabled devices accessing the content.

Note: This technique removes all scripts, styles, and plugins, but it is important to state that this is not required for conformance with WCAG 2.0. An author could use a similar technique, but retain a reduced set of styles and scripts in the “pre-enhanced” version.

While this technique offers a way to base conformance claims on a single version, authors should continue to work to ensure that each enhanced version of the web page is as conformant as possible.


Example 1: Using JavaScript

The example uses JavaScript in the "accToggle.js" file to store the initial pre-enhanced version of the web page, created solely from the HTML in the source code, so that it can act as a "conforming alternate version" for any later enhanced versions of the web page; and inserts a toggle link into all enhanced versions of the web page which allows a user to revert the web page back to the stored pre-enhanced "Conforming Alternate Version". Note: The "sayhello.js" file is simply there as an example payload external file, and is to be replaced by any other external scripts which are desired.

The script in the acctoggle.js file stores the pre-enhanced version - assigning the version the url postfix #accessible. Clicking the "WCAG 2.0 conforming alternate version" link (inserted as the first child of the body element in any enhanced versions) changes the url to include the postfix "#accessible" which then resets the html located in the body element and the head element to pre-enhanced code. The pre-enhanced state can be reached from the link, or directly from a url typed into the browser. In addition, a link is inserted into the pre-enhanced "Conforming Alternate Version" which allows the user to re-enhance the web page (something which can also be done using the web browser's back button).

acctoggle.js source code:

window.onload = function(event) { // store pre-enhanced element content var initialHead = document.head.innerHTML; var initialBody = document.body.innerHTML; var initialURL = location.href; var runOnce = function() { // payload you want to run per page call - e.g. Google Analytics code } var setup = function() { // create conforming alternate version link var toggleEnhanced = document.querySelector("#toggle_enhanced"); if (toggleEnhanced) { toggleEnhanced.outerHTML = ""; } var nel = document.createElement("a"); = "acctoggle"; nel.setAttribute("href", "#accessible"); nel.innerHTML = "WCAG 2.0 conforming alternate version"; document.body.insertBefore(nel, document.body.firstChild); // payload var s = document.createElement("SCRIPT"); s.setAttribute("src", "sayhello.js"); document.querySelector("HEAD").appendChild(s); } setup(); runOnce(); window.onpopstate = function(event) { if (location.href.indexOf("#accessible") != -1) { // revert element contents to pre-enhanced version document.head.innerHTML = initialHead; document.body.innerHTML = initialBody; // create enhanced version link var el = document.createElement("a"); = "toggle_enhanced"; el.setAttribute("href", ""); el.innerHTML = "Enhanced version"; var back = function(e) { e.preventDefault(); window.history.back(); } el.addEventListener("click", back, false); document.body.insertBefore(el, document.body.firstChild); } if (location.href == initialURL) { setup(); } }; }

HTML web page source code:

<!DOCTYPE html> <html lang="en"> <head> <title>Evaluera Ltd</title> <meta charset="UTF-8" /> <script src="accSwitch.js"></script> </head> <body> <h1>Test Page</h1> <p>Say: <span id="change">Goodbye</span></p> </body> </html>

sayhello.js source code

var change = document.querySelector("#change"); change.innerText = "Hello";

Example 2: Using EnhanceJS - A Javascript framework designed to improve the application of Progressive Enhancement

EnhanceJS is an open source JavaScript framework "designed to improve the application of Progressive Enhancement by first testing browser capabilities for key Javascript and CSS support before applying advanced styles and scripts to the page". In addition, the default EnhanceJS script automatically creates a toggle link in any post-enhanced versions of the page which allows a user to return the web page to its pre-enhanced state (in EnhanceJS with default settings this is called the "low bandwidth version"). The setting have been changed in EnhanceJS to indicate that the pre-enhanced state is to be considered the "WCAG 2.0 conforming alternate version", rather than the "low bandwidth version".

HTML Component:

<!DOCTYPE html> <html lang="en"> <head> <script type="text/javascript" src="enhance.js"></script> <script type="text/javascript"> // Run capabilities test enhance({ loadStyles: [ "example.css" ], loadScripts: [ "example.js" ], // text shown in enhanced mode forceFailText: "WCAG 2.0 conforming alternate version", // text shown in accessible mode forcePassText: "Enhanced version" }); </script> </head> ....


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  1. Check enhanced versions of the web page contain a link to the "Conforming Alternate Version".

  2. Check that the alternate version is a conforming alternate version of the original page and that it conforms to WCAG 2.0 at the claimed conformance level.

Expected Results

If this is a sufficient technique for a success criterion, failing this test procedure does not necessarily mean that the success criterion has not been satisfied in some other way, only that this technique has not been successfully implemented and can not be used to claim conformance.