Reason
The namespace is an area, a space, where the tags and attributes in a document's mark-up language are defined by their names. A browser should refer to the namespace in order to interpret the tags and attributes.
The xmlns attribute with its namespace designation is not required when an XHTML document carries a standard XHTML DTD ~ and when the document is composed only in XHTML, not involving any changes of the mark-up language in the page.
While it may not be required for your work, you may want to include the xmlns attribute in the opening <html> tag, just for self-discipline, just to learn to conform further to XML and to prepare yourself further for developments.
