A simple HTML5 document:
The <html> tag tells the browser that this is an HTML document.
The <html> tag represents the root of an HTML document.
The <html> tag is the container for all other HTML elements (except for the <!DOCTYPE> tag).
HTML5 has added a new attribute: manifest.
The xmlns attribute is required in XHTML, but is invalid in HTML.
However, the HTML validator at http://w3.org does not complain when the xmlns attribute is missing in an XHTML document. This is because the namespace "xmlns=http://www.w3.org/1999/xhtml" is default, and will be added to the <html> tag even if you do not include it.
= New in HTML5.
|manifest||URL||Specifies the address of the document's cache manifest (for offline browsing)|
|xmlns||http://www.w3.org/1999/xhtml||Specifies the XML namespace attribute (If you need your content to conform to XHTML)|
The <html> tag also supports the Global Attributes in HTML.
HTML tutorial: HTML Introduction
HTML DOM reference: HTML Object
Most browsers will display the <html> element with the following default values: