Boop is an engine that takes notifications from bash (and similar) scripts and delivers them to any desktop or mobile browser that supports the HTML5 WebPush standard. It’s an open-source alternative to PushBullet.
To display a notification, all you need to do from a source device is:
boop "Title" "Longer text content that can take up more than one line."
This is delivered as:
You can set it up by running a Python script and navigating to the webpage it serves from each browser you want that notification to appear on. There is no configuration beyond adding and removing browsers.
Parts of this script eventually made it into my custom server configuration for Locus Lab.