import { RETAINED_304_HEADERS } from "https://dotland.deno.dev/x/hono@v4.3.10/middleware.ts";

Default headers to pass through on 304 responses. From the spec:

The response must not contain a body and must include the headers that would have been sent in an equivalent 200 OK response: Cache-Control, Content-Location, Date, ETag, Expires, and Vary.