The distinction in my mind is that if a browser doesn’t ship with XSLT, then devs have to go through the hassle of adding support for it themselves, but if a browser doesn’t support a device driver, it’s completely impossible for devs to do that themselves.

Without built-in support, XSLT is inconvenient. Without built-in support, things like WebUSB cannot possibly exist.

That’s why I think they can’t be compared directly.