This month the British Standards Institution finally published the "Publicly Available Specification 78 (PAS 78): Guide to Good Practice in Commissioning Accessible Web Sites".

PAS 78 has been under development by the BSI and the Disability Rights Commission (DRC) for many months.

Its intention is to provide those commissioning and designing web sites with clear guidance as to how to implement accessibility effectively.

The RNIB points out that the disabled market has an estimated spending power of £50 billion a year - so there is a good business case as well as a social case for ensuring that your web site is accessible.

At the moment the most familiar standard for those involved in accessibility is the W3C Web Accessibility Initiative (WAI).

However, WAI takes a very technical approach, covering areas like the use of HTML elements. By contrast PAS 78 has been written from a business viewpoint.

PAS 78 has six principles. These cover the need for an accessibility policy, support for the WAI technical guidelines, guidance on conformance checking, the involvement of disabled people in requirements, design and testing, and the consideration of additional accessibility provision.

PAS78 is more a route-map for website owners than a stand-alone standard. For any specific details it references existing standards such as those of the WAI.

Having spent quite a bit of time over the last few months working with visually impaired users I can whole-heartedly support the importance of involving disa-bled people in web site design.

Whilst there are several automated systems for checking web sites against the WAI criteria (Watchfire's Bobby being the most well known) they are no substitute for a visually impaired user telling you, often quite brutally, where your web site fails.

It is often the most subtle changes in a web site that can help, or hinder, its accessibility.

For instance simply adding "to" or "the" at the front of each menu item can cause real problems for a visually impaired users who is trying to find links by initial letter.

Web site owners also need to bear in mind there are two other factors which will play an important part in the success of their site for disabled users - the quality of any assistive technology being used (screen-readers, magnifiers etc), and the level of IT literacy of the user.

I have met visually impaired users who range from near computer novices to those who use Skype and can hack code with the best of them.

There is always a danger that web site owners see a "generic" disabled person, rather than recognising that disabled individuals will vary as much as able-bodied ones.

Perhaps the most telling part of PAS78 is para 3.9 and its definition of disability as "physical or mental impairment which has a substantial and long-term adverse effect on [a person's] ability to carry out normal day-to-day activities".

* David Burden runs his own information consultancy, Daden Consulting.