At the start of 2012 I published results on the state of HTML5 hidden attribute support and aria-hidden attribute support. Based on the results, advice was provided on the best methods to hide content from some or all users.
Blog posts matching the search: "HTML5 accessibility chops"
ARIA role=alert is supported across modern browsers and assistive technology, but implementation in browsers differ, which can lead to role=alert appearing to be unsupported.
I have jotted down some notes on ARIA concepts and usage that may be useful for developers.