optimization: use client hints for SSR

[yocontra]

https://developers.google.com/web/updates/2015/09/automating-resource-selection-with-client-hints

We can use these new hint headers to get the actual constraints and use those values on the server instead of making an educated guess then doing a re-render on the client. Any user who enables these hints on their page would be able to benefit from the optimization, otherwise the current method still applies.

Would be a good PR!