HTML5 introduces many new features for improving the semantic structure of documents, incorporating multimedia and graphics, and interacting with forms and graphical objects. These include new elements like <video>, <audio>, <canvas>, and <svg> for embedding multimedia and graphics, as well as new form controls. CSS3 also introduces new selectors and properties for effects like rounded corners, shadows, gradients, and transformations. JavaScript APIs allow access to features like geolocation, offline storage, and communication between frames. Browser support for HTML5 features is increasing but not yet complete, so techniques like feature detection and polyfills are recommended.