Although there are no international legal requirements that websites must meet, there are industry standards and national laws regarding accessibility.
The WAI (Web Accessibility Initiative) and XHTML standards are technical guidelines set by the W3C (World Wide Web Consortium), an internationally recognised internet body. These guidelines ensure that a website can be accessed by people in as many different situations as possible, for example people using screen readers and people viewing websites through devices other than a computer such as PDAs.
For UK companies it's not just good practice to follow these guidelines - it's a legal requirement following the Disability Discrimination Act 1995. This requires anyone providing goods, facilities or services to take reasonable steps to allow people with disabilities to make use of them. For websites, this means that any web page available to the public or to employees must be accessible.
Just as importantly, good accessibility allows search engines to index a website more fully and therefore improve its ranking. Headings, titles, descriptions and labels allows further use of keywords and adds richness to a web page.
At Iteracy, we ensure that our website templates meet WAI priority 1 guidelines and are XHTML compliant. Website content entered through a Content Management System can compromise this status, so it's important to educate our clients about their responsibilities, accessibility standards and how to meet them both.
Our policy is to design for the most popular screen size and browsers but to ensure that websites are useable on all modern browsers and operating systems.