Browser cache

__Proposal:__
Use browser cache with http code 304 instead of 200, when a file isn't modified.

__Current behavior:__
The browser download each files again and again at each refresh, ~10 seconds to wait on my server, for example

No files, tests, or entry points are named in the issue; start by locating the HTTP response path that serves files and reviewing how unchanged responses are identified. Done means unchanged files return HTTP 304 responses and refreshes avoid downloading them again, with tests covering that behavior.

Written by the indexing model from the issue text.