Yep, and this can be used to e.g. make a basically static site template and then do an include for `userdata.xml` to decorate your page with the logged in user's info (e.g. on HN, adding your username in the top right, highlighting your comments and showing the edit/delete buttons, etc.). You can for example include into a variable `<xsl:variable name="myinfo" select="document('userdata.xml')"/>` and then use it in xpath expressions like `$myinfo/user/@id`. Extremely simple, good for caching, lightweight, very high performance. Easy to fail gracefully to the logged out template. You basically get your data "API" for free since you're returning XML in your data model. I will never understand why it didn't take off.

XML includes are blocking because XSL support hasn't been updated for 25 years, but there's no reason why we couldn't have it async by now if resources were devoted to this instead of webusb etc.