Thanks to cache partitioning, this is no longer true.
See, it turns out that by carefully leveraging cached retrieval of third party resources, scripts on two different domains can figure out if they are running in the same web browser, effectively pulling off browser fingerprinting.
To stop this, modern browsers don’t share third party resource caches cross domain.
So using a well known CDN no longer confers any benefit - even if a user has pulled down jquery for another site’s benefit, when they come to your site and you request the same resource, the browser will go out and re-retrieve it, to prevent you or the third party domain from being able to infer anything about the user based on the speed of the response or whether or not the request got made.
See, it turns out that by carefully leveraging cached retrieval of third party resources, scripts on two different domains can figure out if they are running in the same web browser, effectively pulling off browser fingerprinting.
To stop this, modern browsers don’t share third party resource caches cross domain.
So using a well known CDN no longer confers any benefit - even if a user has pulled down jquery for another site’s benefit, when they come to your site and you request the same resource, the browser will go out and re-retrieve it, to prevent you or the third party domain from being able to infer anything about the user based on the speed of the response or whether or not the request got made.