What's it all about?
When developing web applications, it's important that all users can use the application; those with low vision, those with no vision, those who are colour-blind, those who have mobility issues and can't use a mouse, those using a touch device and can't use a keyboard, and the temporarily-abled; ie: the rest of us.
The easiest disability to code for is, probably, colour-blindness. Just don't use colour alone to convey information. The most difficult disability would be blindness because you have to take measures to ensure that all information is presented to the user in an understandable way - usually text. So, all images that convey information must have meaningful and appropriate alternate text. Make sure instructions are clear and don't include references to visual cues.
This tutorial will attempt te teach how to write a form that will be accessible to all people.