"Synchronous XMLHttpRequest on the main thread is deprecated" using nodejs app.get or http-server
Thanks for the comments. I have tracked down the issue, and it is within Chrome - more particularly extensions.
The issue is with an extension I have: PropertyWizza v2.0. Disabling this extension clears the problem. I will now uninstall it so it doesn't interfere with my development messages.
This was all deduced because I noticed that I had the same problem when accessing any website - including BBC and GitHub for instance - from any computer in my home, as long as I used Chrome.
My advice to anyone starting to debug front-ends for the first time is to check what errors you get on other websites in all your browsers before you begin. This will provide the "control" for your testing.