To demonstrate the CORS issue, we will look at a simple example where we want to load the www.google.com HTML page and display the HTML in an alert box:
This webpage contains that code. Open that page.
Firefox gives the following error:
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://www.google.com/. (Reason: CORS header 'Access-Control-Allow-Origin' missing).
Now try to open this page, which works.
It uses a CORSified link for www.google.com. QUIPSlab actually opens the page server-side and then sends it to your browser, putting the correct CORS headers whch makes your browser happy.