Please add an additional configuration layer for varnish to give the ability to the user to decide whether to service from varnish cache or the webserver depending on the values stated in the HTTP request header. This can be done with the W3TotalCache plugin (cookie-based caching - maintains separate caches for cookies)

For i.e. If the website is accessed by a mobile device (can be determined by the request header), serve from the mobile cache. If not, serve it from a different cache, etc.

This is very important if the website is specially optimized for mobiles compared to its counter part desktop website.

Google page insight reports having too many html tags (DOM depth, DOM length) for mobile pages even if you use responsive design to go from desktop website to mobile website. So what WordPress developers can do is, remove the desktop load by removing the HTML tags that are not neeed for the mobile website. But in order to do this, varnish cache should be able to have multiple caches.

This is easily achievable in wordpress if you use W3TotalCache plugin. But varnish is a reverse proxy cache which we will have to configure separately.

Thank you.