What influences accessibility of web pages? Research was performed on how the code generated by an authoring tool and the HTML-tags and attributes present in a document influence accessibility. In addition, the impact of the capabilities of hard- and software deployed on the client side as well as the capabilities of the person who actually tries to use the information was analyzed. Based on the findings, a method was developed to measure accessibility of web pages and a list of recommendations was compiled to create documents that are accessible by virtually everyone.

Keywords: Accessibility, Measurement, Guidelines, Web Accessibility Initiative (WAI) (5)

Tim Berners Lee - often referred to as the "father of the Web" - once said: "The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect"(5). To underline this, the W3C has started the "Web Accessibility Initiative" (WAI) (5) in 1997. In the future, there will be an increasing number of "light clients", such as Windows CE based systems, handheld devices, watches, phones and pagers with built-in browsers, which may not be able to support client-based scripting or large images. Authors should try to create documents that are useful for everyone, regardless of the clients hard- and software, the physical, sensory or cognitive capabilities of the user or the user's economic, experiential, equipment or time disadvantages.

Accessibility is defined as how easy a document on the web can be "used" by a client. The higher the requirements are, the less accessible a document is. Requirements include capabilities of the hard- and software deployed as well as the capabilities of the person trying to use the information provided in a document. Accessibility does not mean minimal page design; it means thoughtful page design [A]. Web technology is exclusive or excluding when it limits users' experience or access to information [B]. Excluding sites are seen as sign of corporate incompetence [C].

• Findability: Visitors must be able to find a web-site, either through an easy to remember address or through search engine registration and good TITLE and META-tags. Noframe-information and a good linking strategy are also important.
• Valuable information: It's the content that counts, not the appearance.
• Accessibility: It must be easy to access the information, without plug-ins or special hard- or software. Highly structured documents are more accessible, since the HTML-designers intended HTML to organize content, not to present it. High accessibility indicates that the author cares about people with older browsers, slow access or disabilities.

The following properties of the hardware have an impact on accessibility:

• Resolution of the screen: Some layouts depend on a minimal window width. Fixed width attributes of tables or frames may lead to inaccessible parts of a document on screens with low resolution.
• Number of colors: The lower the number of supported colors, the less distinguishable similar colors may become. In addition, some people may have difficulties seeing certain colors or color combinations.
• Connection Speed: Web users are impatient [C] and 55% of the users are connected with modem-speed [B].
• Sound card: Without it, audio parts of web-pages are inaccessible.

By definition, browsers should ignore everything they do not understand or they don't know how to handle. But especially the more recent versions of the well known browsers try to display as much information as possible. This is not necessarily bad, however it may invite authors - and programmers of authoring tools - to neglect the correct specifications of HTML (6).

Many web sites depend on Java scripts or applets, but some browsers are not able to execute such scripts, including Microsoft Pocket Internet Explorer on Windows CE and most non-graphical browsers, such as Lynx or WebSpeak. Also, some people may have configured their browser to disallow the execution of any scripts locally for security reasons. With the increasing number of "light browsers", such as Windows CE based clients e.g. built into a car stereo unit, it may become more important to provide non-script web sites. From an accessibility point of view, server side scripting, such as Active Server Pages (ASP) are a good alternative to client side scripting.

To measure accessibility, web pages have been tested with different clients (see fig.1: Browsers). An accessibility value in percent was calculated depending on how much of the web pages actually could be used with each browser ("content-accessibility" c) and depending on each browsers capabilities (fB).

Let:

a = accessibility value in percentages

c = content-accessibility: the information is: fully accessible: [100%], still useful: [67%], important information is missing: [33%], not accessible: [0%]

fB = browser-factor: Fig.1: Browsers (1)

fC= "fixed color scheme"-factor: No: [1], Yes: [0.9]

fI = "text as images"-factor: No: [1], Yes: [0.8]

s = minimal screen width of the web page in pixels. Heuristically 640 pixels (VGA) has been chosen as an acceptable screen width without loss of accessibility. The formula leads to a decrease of the accessibility by 10% if a minimal screen width of 1024 pixels (XGA) is required.

then:

Fig. 2: Calculation of the accessibility value a (1)

Fig. 3: Accessibility-ranking (1) errors = disability access problems (accessibility errors) in Bobby (7). See also the complete list of results.

Main problems in terms of accessibility:

• Scripts: Web sites which depend completely on client-side scripts are hard to access with non-graphical or "light" browsers. If the execution of client-side scripts is required for navigation, it is nearly impossible to move around with non-graphical browsers.
• Graphical Information without alternate tags: e.g. images, image maps or animations. HTML 4 requires the ALT attribute for an IMG tag.
• Text as images: 1. Images do not scale. 2. Images are good for graphical browsers only. 3. Images will not be indexed by search engines. 4. Images slow down the transfer of a document. 5. Images are more difficult to maintain.
• Fixed color schemes: Some people have difficulties to see some particular colors or color combinations. In addition, colors appear different from monitor to monitor.
• Abuse of HTML structural elements for layout: Cascading Style Sheets (CSS) is designed for presentation and formatting.

• Design also for users with slow modems, text- or audio-browsers and any type of accommodative technology. [B]
• Use the ALT attribute in every IMG tag.
• Use the ABBR tag: Abbreviations are especially hard for speech browsers.
• Design tables horizontally: in addition cells should have a title attribute.
• Use Checkers: HTML-validators, such as the W3C HTML Validation Service (8), link checkers, spell checkers and accessibility-validators like Bobby (7).
• Test with different browsers on different platforms at different connection speeds on different monitors (size, resolution, colors) and with images turned off [B].

• [A] W. Chisholm, G. Vanderheiden, I. Jacobs: Web Content Accessibility Guidelines (http://www.w3.org/TR/WD-WAI-PAGEAUTH/), W3C Working Draft 26-Feb-1999.
  
  
• [B] P. Vora: Designing Usable Sites (http://www.upassoc.org/html/1998_conference.html), UPA-Conference, Washington DC, USA, June 22-26, 1998.
  
  
• [C] J. Nielsen: Changes in Web Usability Since 1994 (http://www.useit.com/alertbox/9712a.html), Jakob Nielsens's Alertbox for December 1, 1997.

	Reto Ambühler is technical Webmaster of the Swiss Federal Institute of Technology Zürich. He received his degree in electrotechnics in 1981 from the Technikum Winterthur. Home page: http://www.ra.ethz.ch/
	Jakob Lindenmeyer is WWW-Editor of the Swiss Federal Institute of Technology Zürich. He received his masters degree in natural sciences in 1996 from the Swiss Federal Institute of Technology, ETH Zürich. Home page: http://www.lindenmeyer.ch/jakob/

Footnotes:

(1) This is an abridged version, designed to be reproduced in the WWW8-Proceedings. For the full version of the poster, see http://archiv2.weboffice.ethz.ch/www8/fullpaper/

(2) Corresponding Authors. E-mail: weboffice@ethz.ch

(3) http://www.ethz.ch/

(4) http://www.w3.org/WAI/

(5) http://www.w3.org/MarkUp/

(6) http://www.cast.org/bobby/

(7) http://validator.w3.org/

(9) Poster-presentation of this paper as PDF-file [772 KB] under: http://archiv2.weboffice.ethz.ch/www8/poster.pdf

(10) Paper in several formats: Abstract (1.2 kb) | HTML (21 kb) under http://archiv2.weboffice.ethz.ch/www8/ | PDF (Adobe Acrobat) (105 kb) | Postscript (1226 kb) | Full length paper | Poster as PDF (772 KB)

(11) Newspaper-article about this publication (in German): Lindenmeyer, J., Ambühler, R.: Guter Zugang dient allen. Cross-Media-Publishing erschliesst neue Zielgruppen. Neue Zürcher Zeitung (NZZ) Nr. 30, S.74/B4, 6.Feb.2001, Zürich, 2001. [HTML-version], [PDF (Adobe Acrobat)] (17 kb), [Postscript] (50 kb),| [RTF] (15 kb)