HTML5 and CSS3 provide improvements to building the web. HTML5 introduces more semantic tags that improve accessibility and cleaner code. It also provides native support for video, audio, local storage and better interactions. CSS3 enhances presentation. HTML5 is supported across modern browsers and on mobile, allowing responsive design. New features like canvas and WebGL enable graphic effects. Geolocation allows accessing a user's location with permission. HTML5 aims to make the web platform more powerful and flexible.