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 http://www.w3.org/1999/xhtml and other HTML Namespaces 
 
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    HTML 5 Tutorial

    Namespaces in HTML 5

    Converting to HTML 5

    We are working on converting to HTML 5. Please visit http://www.w3.org/1999/xhtml and other HTML Namespaces on the HTML 5 standard version of this site.

    Commonly used namespaces in HTML include:

    http://www.w3.org/XML/1998/namespace
    the XML namespace; implicitly declared
    http://www.w3.org/2000/xmlns/
    the namespace for XML namespaces; also implicitly declared
    http://www.w3.org/1999/xhtml
    the HTML namespace, the same one already being used for XHTML
    http://www.w3.org/1998/Math/MathML
    the MathML namespace
    http://www.w3.org/2000/svg
    the SVG namespace
    the XLink namespace
    http://www.w3.org/2001/XMLSchema-instance
    the namespace for XML Schema instance documents, which can be used to specify whether the data for a field is binary (possibly encrypted) or plain text: 
                                  <span id="masked-credit-card-number" xsi:type="xsd:string">4321 **** **** 8765</span>
                              
    
                              <span id="encrypted-credit-card-number" xsi:type="xsd:base64Binary">BAM0NComFzC2TOsmRzW0NTueQU==</span>
    http://www.w3.org/1999/XSL/Transform
    the namespace for XSL style sheets, not used in HTML documents themselves, but used in the style sheet documents referenced by the xml-stylesheet instruction.

    Namespace Declarations in <html> Tag

    The easiest way to declare namespaces is by putting the xmlns attributes in the top element of the XML document, which in this case is the <html> tag: 

                               <html xmlns="http://www.w3.org/1999/xhtml"
                           
    
                              xmlns:mathml="http://www.w3.org/1998/Math/MathML"
                           
    
                              xmlns:svg="http://www.w3.org/2000/svg"
                           
    
                              xmlns:xlink="http://www.w3.org/1999/xlink"
                           
    
                              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                           
    
                           >
    In HTML 5, all elements (tags) are automatically considered to be qualified with the HTML 5 namespace, which makes the declaration of the HTML namespace optional, but this only works when the HTML parser supports HTML 5 and is actually looking at the page as an HTML version 5 document. It's best to continue coding the xmlns="http://www.w3.org/1999/xhtml" explicitly to provide backward compatibility with non-HTML5-aware browsers and other types of programs that may be parsing the HTML, such as RSS feed readers - otherwise all of the HTML tags will appear to be in the unnamespaced partition.

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    Last updated Sunday June 20, 2010

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