YSlow: "Entity tags (ETags) are a mechanism web servers and the browser use to determine whether a component in the browser's cache matches one on the origin server. Since ETags are typically constructed using attributes that make them unique to a specific server hosting a site, the tags will not match when a browser gets the original component from one server and later tries to validate that component on a different server." More…

Configure ETags using Internet Information Services (IIS) Manager:

Configure ETags in web.config:


                <add name="ETag" value="&quot;&quot;" />

That’s it, now every objet from webserver will have at least one ETag: