HTML 5 is the newest HTML standard used for structuring and presenting content for the Web and a core technology of the Internet. It was created to improve the language with support for the latest multimedia, be easily readable by humans, more logical, and consistently understood by computers and devices such as web browsers, parsers, etc.
HTML5 is intended to replace not only HTML4, but also XHTML1 and DOM Level 2 HTML. Today all major browsers (Chrome, Safari, Firefox, Opera, IE) and many of the mobile ones offer HTML5 support. This is why we improved most of our ASP.NET AJAX controls not only to be compatible with HTML DOCTYPE, but to pass HTML5 validation as well. The controls continue to have a fallback to support earlier technologies and legacy browsers.
You can refer to the table inside this help article to see which controls offer modern rendering and pass HTML5 validation.