The Disability Discrimination Act 1995 (DDA) makes it illegal for a website provider to discriminate against a disabled person. In practice this means you must design your website so that disabled people can access it using technology - eg screen readers.
The World Wide Web Consortium (W3C) has produced a number of accepted guidelines, which are divided into three priorities:
priority 1 - websites must comply otherwise some users will find it impossible to access the site
priority 2 - websites should comply, otherwise some users will find it difficult to access the site
priority 3 - websites may comply, otherwise some users will find it somewhat difficult to access the site
The DDA requires that, where a business puts a disabled person at a substantial disadvantage compared to people who are not disabled, the business must make 'reasonable adjustments'. Therefore, the website owner must:
change a practice, policy or procedure that makes it impossible or unreasonably difficult for disabled people to use the service - eg using very small text that puts vision-impaired people at a disadvantage
provide an auxiliary aid or service if it would enable (or make it easier) for disabled people to use the service - eg offering an alternative point and click interface for visitors that can't use a keyboard
Making websites accessible
There are many adjustments you can make to your website to make it easier for disabled people to use. For example, your web designer can attach a text description to every image and link on the site. This means that vision-impaired users, who rely on a browser that reads out the contents of the webpage, can hear a description of what an image is and where a link will take them. You can read about accessible design on the W3C website.
Checking if your site is accessible
There are a number of tools and services that can help you check that your site is accessible. You can read about and search for accessibility tools on the W3C website.