II. Technical Challenges (Why This Is Not "Just Add onerror")

Dimension	Challenge
Semantics	Must satisfy: try initial URL first, circuit mainly constrains fallback hosts, unmatched resources must not change semantics (still `giveup`, but different from entering the fallback chain).
Vite	Rewriting dynamic import too early breaks `__vitePreload`/`__vite__mapDeps` and async component CSS; literal dynamic import replacement must happen after the bundle is finalized.
Webpack	Global `error` and `__webpack_require__.l` injection can both see the same failure — without DOM markers, double retry; async CSS chunks (mini-css-extract, `experiments.css`, non-`j` loaders in `__webpack_require__.f`) reject promises — Observer replacing `<link>` alone leaves `Promise.all` short-circuited: JS fallback succeeds but lazy load still throws ChunkLoadError.
ESM	Failed module record cache causes "looks like fallback but Network is flat"; same URL + query strategy must be consistent across all entry points.
Config vs reality	When `base`/`publicPath` and rule `match` diverge, Node-side mis-matching arbitrary chunk filenames can rewrite async chunks to wrong CDN — product and engineering misjudgment; needs a `shouldRewriteUrls` gate.
Legacy	SystemJS and Observer without mutually exclusive URL registration → double fallback or missed handling.

II. Technical Challenges (Why This Is Not "Just Add onerror") ​