XML : What is the default namespace for HTML / HTML5?

According to this page by some user named "w3c" the default namespace for HTML is:

  http://www.w3.org/1999/xhtml    

He's obviously wrong since xhtml was used for a failed XML based HTML4 standard. What is the correct namespace I should use?

Background: I'm writing an app that uses XML. I want to be able to save data on the XML using namespaces. For example,

  <s:Button width="100" height="100" html:color="blue" color="black" />     

The XML parser needs a namespace for "html" for it to be valid.