title: should I change <html lang="en_US"> to <html lang="en">？
body: <p>I used to think that if I want to target US audience I should use:</p>
  
  <p><code>&lt;html lang=en_us&gt;</code></p>
  
  <p>However, recently I got this message from <a href="https://validator.w3.org" rel="nofollow noreferrer">https://validator.w3.org</a></p>
  
  <p><code>Error: Bad value en_US for attribute lang on element html: The language subtag en_us is not a valid language subtag.</code></p>
  
  <p>So is the <code>en_US</code> totally wrong? I should change all my html tags to <code>en</code> from <code>en_US</code> now?</p>
  
  <p>Any detailed information on this?</p>