Why there is no fragment negotiation

A "smaller" fragment response only removes the shell markup, which gzip already compresses to almost nothing — while making the same URL return two different bodies, so it needs a Vary header that fragments the cache.

It would also force every fragment to re-include its island <script type="module"> tags and <title>, or navigated-to islands go inert.

The cost that actually matters is the round-trip, not the byte count — so the effort goes into prefetch, and the server stays a plain, cacheable HTML server.