Accessibility checklist

These are the guidelines/requirements based on WCAG 2, but categorised by interaction style. The numbers show the WCAG 2 checkpoint, click through for more information and the official terminology.

Keyboard Use

Check Pass? Checkpoints
The sequence of content makes sense. 1.3.2, 2.4.3
Navigation is consistently placed. 3.2.3
There is a mechanism to skip to content, or skip past navigation. 2.4.1
The location of the keyboard focus is visually obvious. 2.4.7
All functionality is operable with a keyboard. 2.1.1
Keyboard focus does not get trapped. 2.1.2
Changing a widget (e.g. drop down) doesn't automatically take you somewhere else. 3.2.2

Magnification

Check Pass? Checkpoints
The effects of user-action are apparent close to the mouse/keyboard focus (e.g. pop-ups). NA
There is decent contrast for reading text. 1.4.3
You can zoom in 200% without loosing content / functionality. 1.4.4
Graphics are not used for text (except logos). 1.4.5
Check Pass? Checkpoints
There are alternatives for all non-text content. 1.1.1
Audio is not played automatically 1.4.2
When there is multimedia you can work out what is going on from the audio or alternatives. 1.2.*
The HTML structure & semantics reflects the intent. 1.3.1, 2.4.6
Don't rely on colour or location. E.g. "press the red button on the right". 1.3.3, 1.4.1
You can tell where a link takes you. 2.4.4
Form controls are properly labelled, and error messages are available by the relevant input. 3.3.1, 3.3.2
Page titles are unique and useful. 2.4.2
Use standard controls, or ensure that non-standard controls are properly marked up using WAI-ARIA and thoroughly tested. 4.1.2

Deafness & Cognitive Issues

Check Pass? Checkpoints
The content is easy to understand (and explains any jargon). 3.1.3, 3.1.4
Multimedia: You can work out what is going on from the visuals or alternatives. 1.2.*
Any distracting flashes or movements are under user-control. 2.2.2
Terms used for functionality are consistently used. 3.2.4
Error messages are actually useful, e.g. provide a useful next step or way of entering valid data. 3.3.3
There is time to read and act on things. 2.2.1

If you need more detail, you probably should read the full WCAG guidelines.