A bit of vanilla JavaScript with WebComponents is a few lines:
https://gomakethings.com/html-includes-with-web-components/
Edit: “t” was supposed to be the object tag.
You seem to have a rather original definition of "pure HTML".
An html only option that exists is using object. Replying to the miss of the OP in case others might find it suitable.
If a tiny bit of vanilla JavaScript can be tolerated, WebComponents appear to have a broad standardized approach that is not framework dependant.
A bit of vanilla JavaScript with WebComponents is a few lines:
https://gomakethings.com/html-includes-with-web-components/
Edit: “t” was supposed to be the object tag.