Modern HTML lang Attribute for Chinese Websites
Chinese is pretty complicated, same applies to <html lang="__WTF?__"> attribute.
Fortunately, I've already summarized it up:
zh-CN/zh-TW/zh-HK/zh-SGstill works fine, even if there are opposite opinions;zh-cmn-Hans-CNworks best for contents localized for China Mainland users:zhfor backward compatibility since it's been widely adopted by mainstream websites and crawlers, etc;cmndenotes Chinese Mandarin, the official speech of Chinese;Hansdenotes that the contents are in Simplified Chinese form;CNdenotes the contents were specifically adapted for China Mainland.
zh-Hantworks great for contents written in Traditional Chinese;- When in doubt, use bare 
zh. 
Example
<!DOCTYPE html>
<html lang="zh-cmn-Hans-CN">  <meta charset="UTF-8"/>
  <title>你好,世界!</title>
  ...

