Maybe this was mentioned in the context of embedding third-party resources via CDNs? Until recently people would e.g. often include jQuery from a CDN host, and

Maybe this was mentioned in the context of embedding third-party resources via CDNs? Until recently people would e.g. often include jQuery from a CDN host, and given that many websites use the same jQuery script browsers would have that file in the cache already, making the site load faster as the browser can use the local version of the file instead of loading it again. Recently browsers started partitioning caches on a domain level, i.e. each second-level domain has it's own cache, so even if jQuery is cached for foo.com the browser won't use that cache if bar.com also requests that exact same file. The reason for that is that people started abusing browser caches for tracking. That said CDNs are still very useful for a range of functionality such as geographic distribution of content, reducing load to origin servers, DDoS protection and many other applications.