Support Chrome's improved reload behavior or cache-control:immutable
Both Chrome and Firefox have improved the performance of the reload button. Chrome no longer revalidates subresoruces on reloads. Firefox supports the cache-control:immutable header as a way for sites to signal that they do not want their sub resources revalidated before their expiration
Facebook's internal statistics show that browsers that do not support this behavior cause 10-20% more requests for JS/CSS because users use the reload button to refresh the site.
cache-control:immutable shipped with Edge 15 in the recent Windows 10 Creators Update.
Matthias Kurz commented