So having discovered Live.js and Turbowatch and then deciding to write my own simple file watcher using a shell script and Live.js as a reloader, I wondered… do you need to poll?
WebSockets was an option for sending server-side changes to the browser. But too complex for my needs.
Then I found this ancient Server-Side Events API that's been around since Chrome version 6! And it's PERFECT!
https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/