The xmlns label detailed

xmlns attribute

The xmlns attribute can define one or more alternative namespaces in the document. This property can be placed in the start tag of any element within the document. The value of this property is similar to the URL, which defines a namespace that the browser uses for all content within the element that contains the property.

For example, if you need to use an XHTML document that conforms to an XML specification, you should use at least one xmlns attribute in the

 
If you need to display a string of mathematical formulas in a DIV element, you can define a mathematical namespace for that div element. Like this:
 
<div xmlns= "Http://www.w3.org/1999/Math/MathMl" >x3/x</div>
 
If you do not want to define the xmlns attribute in a DIV element each time you display a division formula, it is a better idea to define a namespace with a prefix at the beginning of the document:
 
 
You can then use the prefix in the Div, just like this:
 
<math:div>x3/X<div>
 
Another example is  
<title sec:authentication= "name" ></title> (get login username).