HTML5 is supported in all modern browsers. The latest versions of Google Chrome, Apple Safari, Mozilla Firefox, and Opera all support manyHTML5 features and Internet Explorer 9.0 will also have support for some HTML5 functionality.
The mobile web browsers that come pre-installed on iPhones, iPads, and Android phones all have excellent support for HTML5. In addition, all browsers, old and new, automatically handle unrecognized elements as inline elements.
Because of this, you can “teach” old browsers to handle “unknown” HTML elements.