It's important to distinguish two different formats:
- The RFC 3339/ISO 8601 "wire format": YYYY-MM-DD. According to the HTML5 specification, this is the format that must be used for the input's value upon form submission or when requested via the DOM API. It is locale and region independent.
- The format displayed by the user interface control and accepted as user input. Browser vendors are encouraged to follow the user's preferences selection. For example, on Mac OS with the region "United States" selected in the Language & Text preferences pane, Chrome 20 uses the format "m/d/yy".
The HTML5 specification does not include any means of overriding or manually specifying either format.