[pferde]

Couldn't the same thing be achieved by routing e.g. google.com/static/ to a separate simple webserver, instead of using another domain? Or use a subdomain, e.g. static.google.com.

The current way seems like needless DNS spam to me...

[dndvr]

Even if Google used a separate highly optimised webserver for google.com/static/jquery.js, users who are logged in would be sending their auth cookies when requesting the library.

Given that generally people have slower upload than download, shaving off a few bytes from requests is worth it.

I also recall that browsers [used to (?)] limit concurrent requests per domain which this helps work around