The aim of WAI-ARIA is principally to solve the problem of making dynamic content on a page accessible. It provides a means of describing roles, states, and properties for custom widgets (dynamic sections in web applications) so that they are recognizable and usable by assistive technology users.
For example, if an onscreen widget displays a constantly updating stock price, how would a blind user accessing the page know that? WAI-ARIA attempts to solve this problem. Fully implementing ARIA is outside the scope of this book (for full information, head over to http://www.w3.org/WAI/intro/aria). However, there are some very easy to implement parts of ARIA that we can adopt to enhance any site written in HTML5 for users of assistive technologies.
If you're tasked with building a website for a client, there often isn't any time/money set aside for adding accessibility support beyond the basics (sadly, it's often given no thought at all). However,...