title: What is the difference between <html lang="en"> and <html lang="en-US">?
body: <p>What is the difference between <code>&lt;html lang="en"&gt;</code> and <code>&lt;html lang="en-US"&gt;</code>? What other values can follow the dash?</p>
  
  <p>According to <a href="http://www.w3.org/TR/html4/struct/dirlang.html#langcodes">w3.org</a> "Any two-letter subcode is understood to be a [ISO3166] country code." so does that mean any <a href="http://en.wikipedia.org/wiki/ISO_3166-1#Current_codes">value listed under the alpha-2 code</a> is an accepted value?</p>