The Conservative Party and the Scottish National Party achieved the minimum three stars (on the computing and disability charity's five star scale) required to meet the needs of visitors with a vision impairment, dyslexia or physical problem making mouse use difficult.
Of the other seven sites evaluated by the computing and disability charity for both usability and accessibility, the Liberal Democrats, Plaid Cymru, the SDLP, Sinn Feinn, UKIP and the Ulster Unionist Party achieved only two stars, while the Ulster Democratic Unionist Party scored only one.
Out of the 10 parties, four – Labour, the Conservatives, the Lib Dems and the SNP - have made a public commitment to accessibility, following a request by AbilityNet.
"For the millions of people with a disability or dyslexia considering their electoral choices this Spring, the presence of party information on-line may fulfil a crucial role in the decision making process," said Robin Christopherson, AbilityNet's Web Consultancy Manager, himself blind.
"Whilst no site would knowingly impose a 'technological lock-out' on its disabled visitors, it is clear that there is still much scope for improvement for many of the parties reviewed in this survey," he added.
In March this year the Government published its Digital Strategy, acknowledging the evidence of a 'digital divide' and pointing to the 'barriers to accessibility' exhibited by some web sites as an example of this.
The Strategy points out that web sites, just like other suppliers of services or information, are required to take 'reasonable steps' to ensure accessibility under the provisions of the Disability Discrimination Act.
Christopherson praised the Labour party for practicing what it preached.
"The Labour Party is to be congratulated for setting a precedent for its rivals and we look forward to seeing the Government's Digital Strategy promoting best practice in future web site development," he said.
During the survey, Christopherson and his team encountered the same problems as in previous studies.
Text size on some sites, particularly for headings and links is 'hard-coded' so that it cannot be easily enlarged – so vital for many visitors who have a vision impairment. With some sites offering small text and others carrying a watermark, effective access for this group is made very difficult.
The text labels attached to images upon which blind visitors and text browser users rely for an explanation are often uninformative or completely absent. Without these spoken labels on graphical links, navigation for a blind visitor is pure guesswork. "Imagine trying to drive to your destination where exits at each junction are left blank," said Christopherson.
Pictures of text are often used instead of actual text. This not only means that the user cannot modify the text size or colour contrast – essential for those with a vision impairment or dyslexia – it also prevents screen reader users from reading the content when – as so frequently happens – these images are left unlabelled.
Some sites contain adverts and features made up of moving images that will be distracting for visitors with a cognitive impairment, or interactive presentations known as 'Flash Movies' which can present access problems for visitors who cannot use a mouse, are vision impaired or who use speech output or voice recognition software.
Some of the sites are reliant on mini programs embedded in the page called JavaScript. People using older browsers, those with vision impairments using some special browsers and users whose organisations disable JavaScript for security reasons, will not be able to access the sites fully - links to the main sections do not appear or the search and shopping cart facilities do not operate fully.
These sites exclude a hugely valuable potential market comprising an estimated 2 million vision impaired users, 1.5 million with cognitive difficulties, a further 3.4 million with disabilities preventing them from using the standard keyboard, screen and mouse set-up with ease, around 6 million with dyslexia and many millions with literacy difficulties, not to mention the increasing number of elderly 'silver surfers' with failing eyesight or arthritis.
These potential internet users also represent a spending power in excess of £120 billion, according to AbilityNet.
Adds Christopherson:
"We are now beginning to see examples of highly professional and accessible sites that prove incontrovertibly that an organisation's web site can and should be accessible to the broadest audience possible.
"The Disability Rights Commission shows that able-bodied visitors also benefit from accessible web sites, finding them easier and quicker to use by a highly significant factor of 35%. When we visit a web site we are seeking critical functionality – namely speed and efficiency – not a life-changing experience. Accessible sites are simply easier and more intuitive to use: they improve productivity for everyone."