不成对的 HTML tag

• 
  
  If you are outputting HTML on a regular website you can use
  or
  , both are valid anytime you are serving HTML5 as text/html. If you are serving HTML5 as XHTML (i.e. content type application/xhtml+xml, with an XML declaration) then you must use a self closing tag like so:
  

• ---

  
  In HTML5, the

  ---

  tag defines a thematic break.
  In HTML 4.01, the

  ---

  tag represents a horizontal rule.
  However, the

  ---

  tag may still be displayed as a horizontal rule in visual browsers.