When a browser addresses a page, the server assigns an ETag to that page. This ETag is used by the browser to cache the page's contents. Upon subsequent requests, the browser sends the ETag to the corresponding server, thus letting the server learn the visitor’s identity. As long as the site's files are cached, the ETag is sent every time your browser addresses this site. If the site has embedded content from another server (such as an image or iframe), that server can track your activities without your knowledge.
So I hope you develop this feature and enable it by default. Privacy-related features should be enabled by default