(copied from WCAG specification)

WCAG 2.0 Guidelines

1 Perceivable

1.1 Provide text alternatives for any non-text content so that it can be changed into other forms people need, such as large print, braille, speech, symbols or simpler language.
1.2 Provide alternatives for time-based media.
1.3 Create content that can be presented in different ways (for example simpler layout) without losing information or structure.
1.4 Make it easier for users to see and hear content including separating foreground from background.

2 Operable

2.1 Make all functionality available from a keyboard.
2.2 Provide users enough time to read and use content.
2.3 Do not design content in a way that is known to cause seizures.
2.4 Provide ways to help users navigate, find content, and determine where they are.

3 Understandable

3.1 Make text content readable and understandable.
3.2 Make Web pages appear and operate in predictable ways.
3.3 Help users avoid and correct mistakes.

4 Robust

4.1 Maximize compatibility with current and future user agents, including assistive technologies.

from building-navigator.