title: HTML5 <html> attributes xmlns, lang, xml:lang
body: <p>I don't understand the <a href="http://www.whatwg.org/specs/web-apps/current-work/#the-lang-and-xml:lang-attributes" rel="nofollow noreferrer">HTML5 specifications for the <code>lang</code> and <code>xml:lang</code> attributes</a> of the opening <code>&lt;html&gt;</code> tag. Scrolling up a bit, I understand that <code>xmlns</code> is a "talisman" (has no effect), but what about <code>lang</code> and <code>xml:lang</code>? Should they be used? If so, what should they be set to?</p>