[pinning a specific top level resource] - You can include an SRI link on one of your pages that points to the top level index.html page on someone else's domain

[pinning a specific top level resource] - You can include an SRI link on one of your pages that points to the top level index.html page on someone else's domain, and the browser will verify the pinned hash of the index.html file. Try it and see. If their index page also uses SRI to pin its own resources, the entire site is pinned. [just unilateraly pin versions of websites?] - If the other website is advertising itself as 'not supposed to change', then yes, this is a way of confirming that it has not changed.