From http://www.w3schools.com (Copyright Refsnes Data)

XForms Namespace

You should use an XForms namespace in HTML and XHTML 1.0.

But hopefully not in XHTML 2.0.

The XForms Namespace

The official namespace for XForms is: http://www.w3.org/2002/xforms

If you want to use XForms in HTML (or XHTML 1.0), you should declare all XForms elements with an XForms namespace.

XForms is expected to be a standard part of XHTML 2.0, eliminating the need for the XForms namespace.

This example uses the XForms namespace:

<html xmlns:xf="http://www.w3.org/2002/xforms">

<head>
<xf:model>
  <xf:instance>
  <person>
    <fname/>
    <lname/>
  </person>
  </xf:instance>
  <xf:submission id="form1" method="get"
  action="submit.asp"/>
</xf:model>
</head>

<body>
<xf:input ref="fname">
<xf:label>First Name</xf:label></xf:input>
<br />
<xf:input ref="lname">
<xf:label>Last Name</xf:label></xf:input>
<br />
<br />
<xf:submit submission="form1">
<xf:label>Submit</xf:label></xf:submit>
</body>
</html>

In the example above we have used the xf: prefix for the XForms namespace, but you are free to call the prefix anything you want.