title: XMLHttpRequest cannot load XXX No 'Access-Control-Allow-Origin' header
body: <h2>tl;dr; About the Same Origin Policy</h2>
  
  <p>I have a Grunt process which initiates an instance of express.js server. This was working absolutely fine up until just now when it started serving a blank page with the following appearing in the error log in the developer's console in Chrome (latest version):</p>
  
  <blockquote>
    <p>XMLHttpRequest cannot load <a href="https://www.example.com/" rel="noreferrer">https://www.example.com/</a>
        No 'Access-Control-Allow-Origin' header is present on the requested 
        resource. Origin '<a href="http://localhost:4300" rel="noreferrer">http://localhost:4300</a>' is therefore not allowed access.</p>
  </blockquote>
  
  <p>What is stopping me from accessing the page?</p>