HTML5 is the new standard for HTML that adds new semantic elements, canvas for drawing, video and audio elements, and form controls like date and color pickers. It is a collaboration between W3C and WHATWG to address changes to the web since 1999. Key features include new elements, canvas drawing, media playback with <video> and <audio> tags, offline web apps, and improved form controls. Browser support is increasing, though not fully implemented. Modernizr can detect support for HTML5 features.