<html> Tag Attributes

Attributes of the <html> tag
global attributes In addition to the local attributes of the <html> tag below, any of the common HTML attributes can also be coded.
xmlns The xmlns attribute is used to declare the namespace URIs and associated prefixes for various namespaces in HTML documents, such as:
xmlns="http://www.w3.org/1999/xhtml"
declares the HTML namespace as the default (no prefix)
xmlns:svg="http://www.w3.org/2000/svg"
declares a namespace URI associated with a namespace prefix, which is svg: in this case

See the HTML Namespace Tutorial for more information on namespaces in HTML.

manifest

a URI reference that resolves to the URL of an application cache manifest, a list of files that can be cached for offline use

For example:

index.html 
<html xmlns="http://www.w3.org/1999/xhtml" manifest="/cache.manifest">
cache.manifest 
# http://www.ExampleOnly.com/cache.manifest
CACHE MANIFEST
NETWORK:
*
CACHE:
/index.html
/styles/style-sheet-screen.css
/styles/style-sheet-handheld.css
FALLBACK:
/ /offline.html

The browser may prompt the user for permission to store the offline content, such as in Firefox:

<html> manifest attribute in Firefox
version The version attribute has been deprecated. Use the xmlns attribute instead.