Using the Right Tag for the Right Job - HTML5 Mastery: Semantics, Standards, and Styling - page 42

this attribute, the essential functional components of a web application (the JavaScript,
for instance) can be cached in the browser so that the page will still function if the user
goes offline and reloads the page.
Note The html element also has an attribute, xmlns , used to declare an XML
namespace, which describes the markup language that is being used. This attribute
should be present in valid XHTML documents. It has only one supported value, the text
http://www.w3.org/1999/xhtml . It is applicable only to XHTML and has ab-
solutely no meaning in HTML, other than to make the transition between HTML and
XHTML easier in a document that may cross over between the two.
Document metadata and scripting elements
After the opening root html tag, the head element's opening tag appears, which con-
tains the document metadata elements that specify such page properties as the title for
the document as it appears in the browser window, a default URL address for elements
on the page, character encoding information, and linked and embedded CSS style sheets,
if they are utilized. Table 2-4 lists the relevant elements along with their content model
categories.