Don't give your link element a closing tag. Either leave it as <link attributes='values'> or make it self closing: <link attributes='values'/>
body{
  background-color:black;
}
<!DOCTYPE html>
<html lang="en">
  <head>
    <link rel="stylesheet" type="text/css" href="css/style.css">
  </head>
  <body>
  </body>
</html>
 
 
Less typing if you leave the / off self closing tags. In HTML5 this is ok, however there are other versions of HTML and XML that require all tags to be formally closed.
link: Are (non-void) self-closing tags valid in HTML5?.
Update: Despite this, most browsers still render correctly when the link element is closed incorrectly. And that wasn't the problem here.