Web Design in a Nutshell, 3rd Edition

Language

Because this Web of ours is “World Wide,” the HTML specifications take into account that documents are published in a variety of languages. For that reason, it is important to identify the language in which the document is written (as in lang="en") and the language of the XML version (as in xml:lang="en"). These attributes are placed in the html root element along with the namespace identifier. The XHTML 1.0 Recommendation suggests you include both attributes in the interest of backward compatibility. See Chapter 6 for a complete list of two-letter language codes.