Normally AT customization happens based on which application currently has focus, and in the case of browsers, which URL is currently loaded. Now we can go inside the document.
We can mark a piece of our UI like this:
<!-- embedded FooComponent_v1 -->
Here's how I imagine it will work: when focus moves within this div, the user agent notices this and fires a platform accessibility event. This event ultimately notifies, for example, a screen reader which then checks for a customization script for FooComponent_v1. This script might offer keyboard operation enhancements, instructions on how to work with a FooComponent_v1 or anything else really.
Like many innovations at the edge, I suspect this will empower exciting and perhaps unintended use cases.
Note: there is discussion about renaming templateid for the final ARIA specification. It is fascinating witnessing the web push the boundaries of how we conceptualize an "application", "module", "component", or "widget".
Thanks for reading.