I always thought that it is not so difficult to find resources about these basics, the recurrence of that question prompted me to finally write my own take on this topic. So here it is, my list of absolute web accessibility basics every web developer should know about.
A very solid overview of basic web accessibility techniques by Marco Zehe.
Check out the W3C/WAI Web Accessibility Tutorials, edited by yours truly with the enormous help of the Education and Outreach Working Group and the accessibility community.(about)