HTML5 and ARIA: Accessibility Benefits and Support

HTML5 and ARIA improve web accessibility, especially for screen reader users. But this benefit depends on the vendors implementing them, the web developers using them, and the technologies that people use to consume them. How does support for basic HTML5 and ARIA vary in current versions of browsers and screen readers? What about older versions? What range of software does your audience use and what are you willing to cater to?  Answering such questions should inform your decision to use these technologies, and help you to understand in advance the consequences of that choice for your users.