https://groups.google.com/a/chromium.org/g/blink-dev/c/rjBSVWAGoHo/m/FhEGrTIkAwAJ?pli=1

"Declaratively control various aspect of HTML fragments:"

  - Sanitize them
  - Bring them from a separate URL (aka "client side include")
  - Buffer them instead of stream

There's an "explainer" doc about how it would work here: https://github.com/WICG/declarative-partial-updates/blob/main/fragment-include-explainer.md