These three are bundled in the question but are separate concepts — two HTTP caching, one DOM.

Cache-Control

An HTTP response header that tells the browser (and proxies/CDNs) whether and how long to cache a response:

• max-age=3600 — fresh for 3600s; serve from cache without asking the server.
• no-cache — may cache, but must revalidate with the server before using.
• no-store — don't cache at all (sensitive data).
• public / private — cacheable by shared caches (CDN) or only the browser.
• immutable — never revalidate (great for content-hashed assets).

It's the primary caching control — it decides freshness.

ETag

An HTTP header that's a fingerprint of the response content (a hash or version token). It powers conditional requests / revalidation:

1. Server sends ETag: "abc123" with a response.
2. Next time, the browser sends If-None-Match: "abc123".
3. If the content is unchanged, the server replies 304 Not Modified with an empty body — the browser reuses its cached copy.
4. If changed, the server sends 200 with the new body and a new ETag.

So ETag saves bandwidth (no re-downloading unchanged content) when the cache needs revalidation. Cache-Control says when to revalidate; ETag makes that revalidation cheap. (Last-Modified/If-Modified-Since is the timestamp-based equivalent.)

DocumentFragment

A completely different thing — a DOM concept. DocumentFragment is a lightweight, minimal container that lives outside the main DOM tree. You build up nodes inside it, then insert it once:

const frag = document.createDocumentFragment();
for (const item of items) {
  const li = document.createElement("li");
  li.textContent = item;
  frag.appendChild(li);        // off-DOM — no reflow per append
}
list.appendChild(frag);        // ONE insertion → ONE reflow; the fragment "dissolves"

Appending to the live DOM 1000 times can trigger reflow each time. Building in a fragment and inserting once triggers a single reflow/repaint. When inserted, the fragment itself isn't added — only its children. It's a batch-DOM-updates performance tool.

The framing

"They're unrelated. Cache-Control is the HTTP header that decides if and how long a response is cached — max-age, no-cache, no-store, immutable. ETag is a content fingerprint for cheap revalidation: the browser sends If-None-Match, and the server can reply 304 Not Modified with no body if nothing changed — saving bandwidth. DocumentFragment is a DOM thing entirely — an off-tree container you batch nodes into and insert once, so 1000 appends cause one reflow instead of 1000."