Here is a very plain and straightforward tutorial created/modified by a Google Employee. The tutorial is broken down into three sections covering JS APIs, HTML and CSS.
It covers JS APIs like New Selectors (getElementsByClassName, querySelectorAll), Web Storage (localStorage), Web SQL Database (openDatabase, transaction, executeSql), Application Cache API (manifest=”cache-manifest”, applicationCache), Web Workers (Worker, onmessage, postMessage), Web Sockets (WebSocket, onopen, postMessage, onmessage, onclose), Notifications (webkitNotifications.checkPermission, webkitNotifications.createNotification, show, webkitNotifications.requestPermission), Drag and Drop (dragstart, event.dataTransfer.setData, event.dataTransfer.effectAllowed) and Geolocation (navigator.geolocation, navigator.geolocation.getCurrentPosition, coords.latitude, coords.longitude).
In HTML it covers New semantic tags, New link relations, Microdata, ARIA attributes, New form field types, Audio, Video, Canvas and Canvas 3D.
Finally, in CSS CSS Selectors, New Font support, Text Wrapping, Columns, Text Stroke, Opacity, HSL color model, Rounded corners, Gradients, Shadows, Background Enhancements, Transitions, Transforms and Animations.
The tutorial is a good starting point to know the capabilities of HTML5 before you go all out to explore.
(Some functionality of the tutorial might work only on Chrome)
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