The preconnect HTML hint allows the application to tell the browser which sockets it will need ahead of initiating the actual requests; preconnect eliminates DNS, TCP, and TLS roundtrips from the request’s critical path.
Using it is easy:
<link rel="preconnect" href="//example.com">
<link rel="preconnect" href="//cdn.example.com" crossorigin>
Currently preconnect is only supported on Firefox and Chrome ( it first appeared in Firefox 39, back in July ). This is a potentially powerful feature, definitely worth experimenting with.