title: AJAX CORS error with Python Flask
body: <p>With my Python Flask application, I wanted to do the following from my Javascript.</p>
  
  <pre><code>&lt;script&gt;
      $.ajax({
          url: "http://localhost:5000/card_clf",
          type: "POST",
          contentType: "application/json",
          data: JSON.stringify({"line": "가나다라.\tstc"}),
          success: function(res) {
              document.getElementById("div01").innerHTML = res;
          }
      });
  &lt;/script&gt;
  </code></pre>
  
  <p>However, I got an error:</p>
  
  <pre><code>Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:5000/card_clf. (Reason: CORS header 'Access-Control-Allow-Origin' missing).
  </code></pre>