The Web Content Accessibility Guidelines (WCAG) have become the international standard for website accessibility. These guidelines are separated into 4 principles: Perceivable, Operable, Understandable, and Robust.
WCAG 2.0 Guidelines and Checklists
The information on your site needs to be available to users in ways that they can perceive. This can be achieved by ensuring content can be experienced via sight, sound and touch.
Your website has to be operable – users need to be able to navigate easily through your site, be able to reach everything via just a keyboard, and not be limited by things like time restraints.
This is exactly like it sounds – your website needs to be understandable to your users. This includes simple, clear and consistent content.
A well-built (robust!) website can successfully interact with assistive technology and multiple browsers. So, whether a person is relying on VoiceOver and Safari, or NVDA and Firefox, the user experience will remain the same.